diff --git a/app/models/iiif_canvas_presenter.rb b/app/models/iiif_canvas_presenter.rb
index fc2b6620d5..7b650248a5 100644
--- a/app/models/iiif_canvas_presenter.rb
+++ b/app/models/iiif_canvas_presenter.rb
@@ -209,7 +209,11 @@ def manifest_attributes(quality, media_type)
     def supplemental_attributes(file)
       if file.is_a?(SupplementalFile)
         label = file.tags.include?('machine_generated') ? file.label + ' (machine generated)' : file.label
-        format = file.file.content_type
+        format = if file.file.content_type == 'text/srt' && file.tags.include?('caption')
+                   'text/vtt'
+                 else
+                   file.file.content_type
+                 end
         language = file.language || 'en'
         filename = file.file.filename.to_s
       else
diff --git a/spec/models/iiif_canvas_presenter_spec.rb b/spec/models/iiif_canvas_presenter_spec.rb
index 345870020e..051d320fa0 100644
--- a/spec/models/iiif_canvas_presenter_spec.rb
+++ b/spec/models/iiif_canvas_presenter_spec.rb
@@ -269,6 +269,16 @@
         expect(subject.any? { |content| content.body_id =~ /master_files\/#{master_file.id}\/captions/ }).to eq false
       end
 
+      context 'srt captions' do
+        let(:srt_caption_file) { FactoryBot.create(:supplemental_file, :with_caption_srt_file, :with_caption_tag) }
+        let(:supplemental_files) { [supplemental_file, transcript_file, caption_file, srt_caption_file] }
+        it 'sets format to "text/vtt"' do
+          captions = subject.select { |s| s.body_id.include?('captions') }
+          expect(captions.none? { |content| content.format == 'text/srt' }).to eq true
+          expect(captions.all? { |content| content.format == 'text/vtt' }).to eq true
+        end
+      end
+
       context 'legacy master file captions' do
         let(:master_file) { FactoryBot.create(:master_file, :with_waveform, :with_captions, supplemental_files_json: supplemental_files_json, media_object: media_object, derivatives: [derivative]) }