diff --git a/CHANGELOG.md b/CHANGELOG.md
index ed20e28..aa13362 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 * raise Vips::Error if `copy_memory` fails [VitaliiLazebnyi]
 * `Source.new_from_memory` keeps a reference to the memory area [jeremy]
+* release the GVL during `copy_memory` and `write_to_memory` [andrewfader]
 
 ## Version 2.3.0 (2025-12-10)
 
diff --git a/lib/vips/image.rb b/lib/vips/image.rb
index af2bd0f..814621e 100644
--- a/lib/vips/image.rb
+++ b/lib/vips/image.rb
@@ -12,7 +12,9 @@ module Vips
   attach_function :vips_image_new_matrix_from_array,
     [:int, :int, :pointer, :int], :pointer
 
-  attach_function :vips_image_copy_memory, [:pointer], :pointer
+  # these two evaluate the whole pipeline, so release the GVL while they run
+  attach_function :vips_image_copy_memory, [:pointer], :pointer,
+    blocking: true
 
   attach_function :vips_image_set_progress, [:pointer, :int], :void
   attach_function :vips_image_set_kill, [:pointer, :int], :void
@@ -31,7 +33,7 @@ module Vips
   end
 
   attach_function :vips_image_write_to_memory,
-    [:pointer, SizeStruct.ptr], :pointer
+    [:pointer, SizeStruct.ptr], :pointer, blocking: true
 
   attach_function :vips_image_get_typeof, [:pointer, :string], :GType
   attach_function :vips_image_get,
diff --git a/spec/image_spec.rb b/spec/image_spec.rb
index f1eeff7..2cb0f4e 100644
--- a/spec/image_spec.rb
+++ b/spec/image_spec.rb
@@ -94,6 +94,36 @@
     expect { img.copy_memory }.to raise_exception(Vips::Error)
   end
 
+  # copy_memory and write_to_memory evaluate the whole pipeline, so they must
+  # release the GVL, or every other Ruby thread stalls until they return
+  def ticks_during
+    ticks = 0
+    running = true
+    ticker = Thread.new do
+      while running
+        ticks += 1
+        sleep 0.001
+      end
+    end
+    sleep 0.01 until ticks > 0
+    before = ticks
+    yield
+    during = ticks - before
+    running = false
+    ticker.join
+    during
+  end
+
+  it "lets other threads run during copy_memory" do
+    image = Vips::Image.gaussnoise(2000, 2000).gaussblur(4)
+    expect(ticks_during { image.copy_memory }).to be > 0
+  end
+
+  it "lets other threads run during write_to_memory" do
+    image = Vips::Image.gaussnoise(2000, 2000).gaussblur(4)
+    expect(ticks_during { image.write_to_memory }).to be > 0
+  end
+
   it "throws an error when trying to load an image from memory with unknown size" do
     data = FFI::Pointer.new(1)
     expect { Vips::Image.new_from_memory(data, 16, 16, 1, :uchar) }.to raise_error(Vips::Error)