diff --git a/CHANGELOG.md b/CHANGELOG.md
index c02ceaa..8960ad3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# HEAD
+
+* Support fiber-local log levels using `Logger#with_level` [#84](https://github.com/ruby/logger/issue/84)
+
 # 1.4.2
 
 * Document that shift_age of 0 disables log file rotation [#43](https://github.com/ruby/logger/pull/43) (thanks to jeremyevans)
diff --git a/lib/logger.rb b/lib/logger.rb
index cafebc5..4940999 100644
--- a/lib/logger.rb
+++ b/lib/logger.rb
@@ -10,6 +10,7 @@
 #
 # A simple system for logging messages.  See Logger for more documentation.
 
+require 'fiber'
 require 'monitor'
 require 'rbconfig'
 
@@ -380,7 +381,9 @@ class Logger
   include Severity
 
   # Logging severity threshold (e.g. <tt>Logger::INFO</tt>).
-  attr_reader :level
+  def level
+    @level_override[Fiber.current] || @level
+  end
 
   # Sets the log level; returns +severity+.
   # See {Log Level}[rdoc-ref:Logger@Log+Level].
@@ -395,24 +398,23 @@ class Logger
   # Logger#sev_threshold= is an alias for Logger#level=.
   #
   def level=(severity)
-    if severity.is_a?(Integer)
-      @level = severity
-    else
-      case severity.to_s.downcase
-      when 'debug'
-        @level = DEBUG
-      when 'info'
-        @level = INFO
-      when 'warn'
-        @level = WARN
-      when 'error'
-        @level = ERROR
-      when 'fatal'
-        @level = FATAL
-      when 'unknown'
-        @level = UNKNOWN
+    @level = Severity.coerce(severity)
+  end
+
+  # Adjust the log level during the block execution for the current Fiber only
+  #
+  #   logger.with_level(:debug) do
+  #     logger.debug { "Hello" }
+  #   end
+  def with_level(severity)
+    prev, @level_override[Fiber.current] = level, Severity.coerce(severity)
+    begin
+      yield
+    ensure
+      if prev
+        @level_override[Fiber.current] = prev
       else
-        raise ArgumentError, "invalid log level: #{severity}"
+        @level_override.delete(Fiber.current)
       end
     end
   end
@@ -583,6 +585,7 @@ def initialize(logdev, shift_age = 0, shift_size = 1048576, level: DEBUG,
     self.datetime_format = datetime_format
     self.formatter = formatter
     @logdev = nil
+    @level_override = {}
     if logdev && logdev != File::NULL
       @logdev = LogDevice.new(logdev, shift_age: shift_age,
         shift_size: shift_size,
diff --git a/lib/logger/severity.rb b/lib/logger/severity.rb
index b38afb7..e96fb0d 100644
--- a/lib/logger/severity.rb
+++ b/lib/logger/severity.rb
@@ -15,5 +15,24 @@ module Severity
     FATAL = 4
     # An unknown message that should always be logged.
     UNKNOWN = 5
+
+    LEVELS = {
+      "debug" => DEBUG,
+      "info" => INFO,
+      "warn" => WARN,
+      "error" => ERROR,
+      "fatal" => FATAL,
+      "unknown" => UNKNOWN,
+    }
+    private_constant :LEVELS
+
+    def self.coerce(severity)
+      if severity.is_a?(Integer)
+        severity
+      else
+        key = severity.to_s.downcase
+        LEVELS[key] || raise(ArgumentError, "invalid log level: #{severity}")
+      end
+    end
   end
 end
diff --git a/test/logger/test_severity.rb b/test/logger/test_severity.rb
index dad6347..e1069c8 100644
--- a/test/logger/test_severity.rb
+++ b/test/logger/test_severity.rb
@@ -3,6 +3,8 @@
 require 'logger'
 
 class TestLoggerSeverity < Test::Unit::TestCase
+  include Logger::Severity
+
   def test_enum
     logger_levels = Logger.constants
     levels = ["WARN", "UNKNOWN", "INFO", "FATAL", "DEBUG", "ERROR"]
@@ -23,4 +25,34 @@ def test_level_assignment
       assert(logger.level) == Logger::Severity.const_get(level)
     end
   end
+
+  def test_thread_local_level
+    logger = Logger.new(nil)
+    logger.level = INFO # default level
+    other = Logger.new(nil)
+    other.level = ERROR # default level
+
+    assert_equal(other.level, ERROR)
+    logger.with_level(:WARN) do
+      assert_equal(other.level, ERROR)
+      assert_equal(logger.level, WARN)
+
+      logger.with_level(DEBUG) do # verify reentrancy
+        assert_equal(logger.level, DEBUG)
+
+        Thread.new do
+          assert_equal(logger.level, INFO)
+          logger.with_level(:WARN) do
+            assert_equal(other.level, ERROR)
+            assert_equal(logger.level, WARN)
+          end
+          assert_equal(logger.level, INFO)
+        end.join
+
+        assert_equal(logger.level, DEBUG)
+      end
+      assert_equal(logger.level, WARN)
+    end
+    assert_equal(logger.level, INFO)
+  end
 end