diff --git a/.github/workflows/build-test.yml b/.github/workflows/build-test.yml
index 6248c0dd5..c847e607d 100644
--- a/.github/workflows/build-test.yml
+++ b/.github/workflows/build-test.yml
@@ -116,3 +116,66 @@ jobs:
       - name: Run pytest
         run: |
           python -m pytest -vv tests --reruns 10 --reruns-delay 30 --only-rerun "(?i)http|ssl|timeout|expired|connection|socket"
+
+  pyodide-build:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    env:
+      PYODIDE_VERSION: 0.26.2
+      PYODIDE_BUILD_VERSION: 0.28.0
+      AWKWARD_VERSION: v2.6.4
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install pyodide-build
+        run: python3 -m pip install pyodide-build==$PYODIDE_BUILD_VERSION
+
+      - name: Determine EMSDK version
+        id: compute-emsdk-version
+        run: |
+          pyodide config list
+          # Save EMSDK version
+          EMSCRIPTEN_VERSION=$(pyodide config get emscripten_version)
+          echo "emsdk-version=$EMSCRIPTEN_VERSION" >> $GITHUB_OUTPUT
+
+      - name: Install EMSDK
+        uses: mymindstorm/setup-emsdk@v14
+        with:
+          version: ${{ steps.compute-emsdk-version.outputs.emsdk-version }}
+
+      - name: Build the package
+        run: pyodide build
+
+      - name: Build an awkward wheel compatible with the awkward-cpp version in pyodide
+        run: |
+          git clone --depth 1 --branch $AWKWARD_VERSION https://github.com/scikit-hep/awkward.git dependencies/awkward
+          pyodide build dependencies/awkward
+          rm -rf dependencies/
+
+      - name: Download Pyodide
+        uses: pyodide/pyodide-actions/download-pyodide@v1
+        with:
+          version: ${{ env.PYODIDE_VERSION }}
+          to: pyodide-dist
+
+      - name: Install browser
+        uses: pyodide/pyodide-actions/install-browser@v1
+        with:
+          runner: selenium
+          browser: chrome
+          browser-version: latest
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Install dependencies
+        run: pip install .[test-pyodide] pyodide-py==$PYODIDE_VERSION
+
+      - name: Run pytest
+        run: |
+          pytest -vv --dist-dir=./pyodide-dist/ --runner=selenium --runtime=chrome tests-wasm
diff --git a/.gitignore b/.gitignore
index 0aceed8b2..777961eb1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -136,3 +136,10 @@ dmypy.json
 
 # Pyre type checker
 .pyre/
+
+# Local copies of skhep_testdata files
+skhep_testdata/
+
+# Pyodide
+.pyodide*
+dist-pyodide/
diff --git a/pyproject.toml b/pyproject.toml
index 0a5b74418..08cda684e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -80,6 +80,12 @@ test = [
   "scikit-hep-testdata",
   "rangehttpserver"
 ]
+test-pyodide = [
+  "pytest>=6",
+  "pytest-pyodide",
+  "pytest-timeout",
+  "scikit-hep-testdata"
+]
 xrootd = ["fsspec-xrootd"]
 
 [project.urls]
diff --git a/src/uproot/_util.py b/src/uproot/_util.py
index 11c1d5bc4..96e9e8d21 100644
--- a/src/uproot/_util.py
+++ b/src/uproot/_util.py
@@ -13,6 +13,7 @@
 import numbers
 import os
 import re
+import sys
 import warnings
 from collections.abc import Iterable
 from pathlib import Path
@@ -27,6 +28,8 @@
 import uproot.source.fsspec
 import uproot.source.object
 
+wasm = sys.platform in ("emscripten", "wasi")
+
 
 def tobytes(array):
     """
diff --git a/src/uproot/source/coalesce.py b/src/uproot/source/coalesce.py
index 52cad9289..ab3ada9b7 100644
--- a/src/uproot/source/coalesce.py
+++ b/src/uproot/source/coalesce.py
@@ -33,6 +33,9 @@ def add_done_callback(self, callback, *, context=None):
         self._parent.add_done_callback(callback)
 
     def result(self, timeout=None):
+        if uproot._util.wasm:
+            # Pyodide futures don't support timeout
+            return self._parent.result()[self._s]
         return self._parent.result(timeout=timeout)[self._s]
 
 
@@ -126,7 +129,13 @@ def coalesce_requests(
 
     def chunkify(req: RangeRequest):
         chunk = uproot.source.chunk.Chunk(source, req.start, req.stop, req.future)
-        req.future.add_done_callback(uproot.source.chunk.notifier(chunk, notifications))
+        if uproot._util.wasm:
+            # Callbacks don't work in pyodide yet, so we call the notifier directly
+            uproot.source.chunk.notifier(chunk, notifications)()
+        else:
+            req.future.add_done_callback(
+                uproot.source.chunk.notifier(chunk, notifications)
+            )
         return chunk
 
     return list(map(chunkify, all_requests))
diff --git a/src/uproot/source/fsspec.py b/src/uproot/source/fsspec.py
index 588b6b31d..6f47890ba 100644
--- a/src/uproot/source/fsspec.py
+++ b/src/uproot/source/fsspec.py
@@ -164,6 +164,12 @@ def submit(request_ranges: list[tuple[int, int]]):
                     self._fs.cat_ranges, paths=paths, starts=starts, ends=ends
                 )
             )
+            if uproot._util.wasm:
+                # Threads can't be spawned in pyodide yet, so we run the function directly
+                # and return a future that is already resolved.
+                return uproot.source.futures.TrivialFuture(
+                    self._fs.cat_ranges(paths=paths, starts=starts, ends=ends)
+                )
             return self._executor.submit(coroutine)
 
         return coalesce_requests(
diff --git a/src/uproot/source/http.py b/src/uproot/source/http.py
index 2f0b64a03..e3fb1e617 100644
--- a/src/uproot/source/http.py
+++ b/src/uproot/source/http.py
@@ -239,6 +239,23 @@ def future(source: uproot.source.chunk.Source, start: int, stop: int):
         Returns a :doc:`uproot.source.futures.ResourceFuture` that calls
         :ref:`uproot.source.http.HTTPResource.get` with ``start`` and ``stop``.
         """
+        # The default implementation doesn't work in Pyodide
+        if uproot._util.wasm:
+
+            def task(resource):
+                import requests
+
+                r = requests.get(
+                    source._file_path,
+                    headers=dict(
+                        {"Range": f"bytes={start}-{stop - 1}"}, **source.auth_headers
+                    ),
+                    timeout=source.timeout,
+                )
+                return r.content
+
+            return uproot.source.futures.ResourceFuture(task)
+
         connection = make_connection(source.parsed_url, source.timeout)
         connection.request(
             "GET",
@@ -281,6 +298,14 @@ def multifuture(
         ``results`` and ``futures``. Subsequent attempts would immediately
         use the :ref:`uproot.source.http.HTTPSource.fallback`.
         """
+        # The default implementation doesn't work in Pyodide
+        if uproot._util.wasm:
+
+            def task(resource):
+                resource.handle_no_multipart(source, ranges, futures, results)
+
+            return uproot.source.futures.ResourceFuture(task)
+
         connection = make_connection(source.parsed_url, source.timeout)
 
         connection.request(
diff --git a/tests-wasm/__init__.py b/tests-wasm/__init__.py
new file mode 100644
index 000000000..f8c1f9969
--- /dev/null
+++ b/tests-wasm/__init__.py
@@ -0,0 +1 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
diff --git a/tests-wasm/test_1272_basic_functionality.py b/tests-wasm/test_1272_basic_functionality.py
new file mode 100644
index 000000000..68d14e575
--- /dev/null
+++ b/tests-wasm/test_1272_basic_functionality.py
@@ -0,0 +1,124 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
+from __future__ import annotations
+
+import pytest
+from utils import run_test_in_pyodide
+
+
+# Taken from test_0034_generic_objects_in_ttrees.py
+@run_test_in_pyodide(test_file="uproot-HZZ-objects.root", packages=["pytest", "xxhash"])
+def test_read_ttree(selenium):
+    import pytest
+
+    import uproot
+
+    awkward = pytest.importorskip("awkward")
+
+    with uproot.open("uproot-HZZ-objects.root")["events"] as tree:
+        result = tree["muonp4"].array(library="ak")
+
+        assert (
+            str(awkward.type(result))
+            == "2421 * var * TLorentzVector[fP: TVector3[fX: float64, "
+            "fY: float64, fZ: float64], fE: float64]"
+        )
+
+        assert result[0, 0, "fE"] == 54.77949905395508
+        assert result[0, 0, "fP", "fX"] == -52.89945602416992
+        assert result[0, 0, "fP", "fY"] == -11.654671669006348
+        assert result[0, 0, "fP", "fZ"] == -8.16079330444336
+
+
+# Taken from test_0406_write_a_tree.py
+@run_test_in_pyodide()
+def test_write_ttree(selenium):
+    import numpy as np
+
+    import uproot
+
+    newfile = "newfile.root"
+
+    b1 = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+    b2 = [0.0, 1.1, 2.2, 3.3, 4.4, 5.5, 6.6, 7.7, 8.8, 9.9]
+
+    with uproot.recreate(newfile, compression=None) as fout:
+        tree = fout.mktree("t", {"b1": np.int32, "b2": np.float64}, "title")
+
+        assert tree._cascading._basket_capacity == 10
+
+        for _ in range(5):
+            fout["t"].extend({"b1": b1, "b2": b2})
+
+        assert tree._cascading._basket_capacity == 10
+
+        for _ in range(10):
+            fout["t"].extend({"b1": b1, "b2": b2})
+
+        assert tree._cascading._basket_capacity == 100
+
+        for _ in range(90):
+            fout["t"].extend({"b1": b1, "b2": b2})
+
+        assert tree._cascading._basket_capacity == 1000
+
+    with uproot.open(newfile) as fin:
+        assert fin.keys() == ["t;1"]  # same cycle number
+        t2 = fin["t"]
+        assert t2.num_entries == len(b1) * 105
+        assert t2["b1"].array(library="np").tolist() == b1 * 105
+        assert t2["b2"].array(library="np").tolist() == b2 * 105
+
+
+# Taken from test_1191_rntuple_fixes.py
+@run_test_in_pyodide(test_file="test_ntuple_extension_columns.root")
+def test_read_rntuple(selenium):
+    import uproot
+
+    with uproot.open("test_ntuple_extension_columns.root") as f:
+        obj = f["EventData"]
+
+        assert len(obj.column_records) > len(obj.header.column_records)
+        assert len(obj.column_records) == 936
+        assert obj.column_records[903].first_ele_index == 36
+
+        arrays = obj.arrays()
+
+        pbs = arrays[
+            "HLT_AntiKt4EMPFlowJets_subresjesgscIS_ftf_TLAAux::fastDIPS20211215_pb"
+        ]
+        assert len(pbs) == 40
+        assert all(len(a) == 0 for a in pbs[:36])
+        assert next(i for i, a in enumerate(pbs) if len(a) != 0) == 36
+
+        jets = arrays["HLT_AntiKt4EMPFlowJets_subresjesgscIS_ftf_TLAAux:"]
+        assert len(jets.pt) == len(pbs)
+
+
+# Taken from test_0088_read_with_http.py
+@pytest.mark.network
+@run_test_in_pyodide(packages=["requests"])
+def test_read_ttree_http(selenium):
+    import uproot
+
+    with uproot.open(
+        "http://starterkit.web.cern.ch/starterkit/data/advanced-python-2019/dalitzdata.root",
+        handler=uproot.source.http.HTTPSource,
+    ) as f:
+        data = f["tree"].arrays(["Y1", "Y2"], library="np")
+        assert len(data["Y1"]) == 100000
+        assert len(data["Y2"]) == 100000
+
+
+# Taken from test_1191_rntuple_fixes.py
+@pytest.mark.network
+@run_test_in_pyodide(packages=["requests"])
+def test_read_rntuple_http(selenium):
+    import uproot
+
+    with uproot.open(
+        "https://github.com/scikit-hep/scikit-hep-testdata/raw/main/src/skhep_testdata/data/Run2012BC_DoubleMuParked_Muons_rntuple_1000evts.root",
+        handler=uproot.source.http.HTTPSource,
+    ) as f:
+        obj = f["Events"]
+        arrays = obj.arrays()
+        assert arrays["nMuon"].tolist() == [len(a) for a in arrays["Muon_pt"]]
diff --git a/tests-wasm/utils.py b/tests-wasm/utils.py
new file mode 100644
index 000000000..a703b13e2
--- /dev/null
+++ b/tests-wasm/utils.py
@@ -0,0 +1,63 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/uproot5/blob/main/LICENSE
+from __future__ import annotations
+
+import os
+import pathlib
+import shutil
+from functools import wraps
+
+import pytest
+import skhep_testdata
+
+try:
+    import pytest_pyodide
+    from pytest_pyodide import run_in_pyodide
+    from pytest_pyodide.decorator import copy_files_to_pyodide
+except ImportError:
+    pytest.skip("Pyodide is not available", allow_module_level=True)
+
+# Disable CORS so that we can fetch files for http tests
+# Currently, this can only be done for Chrome
+selenium_config = pytest_pyodide.config.get_global_config()
+selenium_config.set_flags(
+    "chrome",
+    [
+        *selenium_config.get_flags("chrome"),
+        "--disable-web-security",
+        "--disable-site-isolation-trials",
+    ],
+)
+
+
+# copy skhep_testdata files to testdata directory (needed for @copy_files_to_pyodide)
+def ensure_testdata(filename):
+    if not pathlib.Path("skhep_testdata/" + filename).is_file():
+        filepath = skhep_testdata.data_path(filename)
+        os.makedirs("skhep_testdata", exist_ok=True)
+        shutil.copyfile(filepath, "skhep_testdata/" + filename)
+
+
+def run_test_in_pyodide(test_file=None, **kwargs):
+    def decorator(test_func):
+        @wraps(test_func)
+        def wrapper(selenium):
+            if test_file is not None:
+                ensure_testdata(test_file)
+
+            @copy_files_to_pyodide(
+                file_list=[("dist", "dist")]
+                + (
+                    []
+                    if test_file is None
+                    else [("skhep_testdata/" + test_file, test_file)]
+                ),
+                install_wheels=True,
+            )
+            def inner_func(selenium):
+                run_in_pyodide(**kwargs)(test_func)(selenium)
+
+            return inner_func(selenium)
+
+        return wrapper
+
+    return decorator
diff --git a/tests/test_1191_rntuple_fixes.py b/tests/test_1191_rntuple_fixes.py
index 7b64e45d2..a1e259310 100644
--- a/tests/test_1191_rntuple_fixes.py
+++ b/tests/test_1191_rntuple_fixes.py
@@ -41,7 +41,7 @@ def test_rntuple_cardinality():
 def test_skip_recursively_empty_structs():
     filename = skhep_testdata.data_path("DAOD_TRUTH3_RC2.root")
     with uproot.open(filename) as f:
-        obj = uproot.open(filename)["RNT:CollectionTree"]
+        obj = f["RNT:CollectionTree"]
         arrays = obj.arrays()
         jets = arrays["AntiKt4TruthDressedWZJetsAux:"]
         assert len(jets[0].pt) == 5