diff --git a/asdf/asdf.py b/asdf/asdf.py
index 3e43602ec..89cb8eb52 100644
--- a/asdf/asdf.py
+++ b/asdf/asdf.py
@@ -62,7 +62,6 @@ def __init__(
         copy_arrays=False,
         lazy_load=True,
         custom_schema=None,
-        _readonly=False,
     ):
         """
         Parameters
@@ -156,7 +155,7 @@ def __init__(
         self._fd = None
         self._closed = False
         self._external_asdf_by_uri = {}
-        self._blocks = block.BlockManager(self, copy_arrays=copy_arrays, lazy_load=lazy_load, readonly=_readonly)
+        self._blocks = block.BlockManager(self, copy_arrays=copy_arrays, lazy_load=lazy_load)
         self._uri = None
         if tree is None:
             # Bypassing the tree property here, to avoid validating
@@ -1819,13 +1818,10 @@ def open_asdf(
         The new AsdfFile object.
     """
 
-    readonly = False
-
     # For now retain backwards compatibility with the old API behavior,
     # specifically when being called from AsdfFile.open
     if not _compat:
         mode = _check_and_set_mode(fd, mode)
-        readonly = mode == "r" and not copy_arrays
 
     instance = AsdfFile(
         ignore_version_mismatch=ignore_version_mismatch,
@@ -1833,7 +1829,6 @@ def open_asdf(
         copy_arrays=copy_arrays,
         lazy_load=lazy_load,
         custom_schema=custom_schema,
-        _readonly=readonly,
     )
 
     return AsdfFile._open_impl(
diff --git a/asdf/block.py b/asdf/block.py
index 5b51868e0..45d60ade2 100644
--- a/asdf/block.py
+++ b/asdf/block.py
@@ -21,7 +21,7 @@ class BlockManager:
     Manages the `Block`s associated with a ASDF file.
     """
 
-    def __init__(self, asdffile, copy_arrays=False, lazy_load=True, readonly=False):
+    def __init__(self, asdffile, copy_arrays=False, lazy_load=True):
         self._asdffile = weakref.ref(asdffile)
 
         self._internal_blocks = []
@@ -40,7 +40,6 @@ def __init__(self, asdffile, copy_arrays=False, lazy_load=True, readonly=False):
         self._validate_checksums = False
         self._memmap = not copy_arrays
         self._lazy_load = lazy_load
-        self._readonly = readonly
         self._internal_blocks_mapped = False
 
     def __len__(self):
@@ -512,9 +511,7 @@ def read_block_index(self, fd, ctx):
         # It seems we're good to go, so instantiate the UnloadedBlock
         # objects
         for offset in offsets[1:-1]:
-            self._internal_blocks.append(
-                UnloadedBlock(fd, offset, memmap=self.memmap, lazy_load=self.lazy_load, readonly=self._readonly)
-            )
+            self._internal_blocks.append(UnloadedBlock(fd, offset, memmap=self.memmap, lazy_load=self.lazy_load))
 
         # We already read the last block in the file -- no need to read it again
         self._internal_blocks.append(block)
@@ -814,7 +811,6 @@ def __init__(self, data=None, uri=None, array_storage="internal", memmap=True, l
         self._should_memmap = memmap
         self._memmapped = False
         self._lazy_load = lazy_load
-        self._readonly = False
 
         self.update_size()
         self._allocated = self._size
@@ -920,10 +916,6 @@ def output_compression_kwargs(self, config):
     def checksum(self):
         return self._checksum
 
-    @property
-    def readonly(self):
-        return self._readonly
-
     def _set_checksum(self, checksum):
         if checksum == b"\0" * 16:
             self._checksum = None
@@ -1234,7 +1226,7 @@ class UnloadedBlock:
     requested.
     """
 
-    def __init__(self, fd, offset, memmap=True, lazy_load=True, readonly=False):
+    def __init__(self, fd, offset, memmap=True, lazy_load=True):
         self._fd = fd
         self._offset = offset
         self._data = None
@@ -1247,7 +1239,6 @@ def __init__(self, fd, offset, memmap=True, lazy_load=True, readonly=False):
         self._should_memmap = memmap
         self._memmapped = False
         self._lazy_load = lazy_load
-        self._readonly = readonly
 
     def __len__(self):
         self.load()
diff --git a/asdf/tags/core/ndarray.py b/asdf/tags/core/ndarray.py
index 5fb217a2e..ce262390b 100644
--- a/asdf/tags/core/ndarray.py
+++ b/asdf/tags/core/ndarray.py
@@ -264,8 +264,6 @@ def _make_array(self):
 
             self._array = np.ndarray(shape, dtype, block.data, self._offset, self._strides, self._order)
             self._array = self._apply_mask(self._array, self._mask)
-            if block.readonly:
-                self._array.setflags(write=False)
         return self._array
 
     def _apply_mask(self, array, mask):