How to Add Subtitles in iMovie

iMovie has no way to import an SRT file or generate captions: Apple's iMovie guides for Mac and iPhone only describe titles, which you add and time one by one. For more than a few lines, burn the subtitles into the video with another tool and bring the finished MP4 into iMovie, or export from iMovie first and add subtitles after.

At a glance

Imports subtitle files	No — Apple's guides describe titles only, not SRT or other caption files
Built-in auto captions	No speech-to-text feature is described
Export as a file	Not available
Burn into the video	Titles are part of the picture, so they are always in the export
Practical limit	Every line is a title you place and time by hand

Add a line as a title on Mac

Open the movie in the timeline and click Titles above the browser.
Double-click a title style to add it at the playhead, or drag it above a clip.
Double-click the placeholder text in the viewer, type the line, set the font and size above the viewer, and click Apply.
Drag the title's edges in the timeline so it lasts as long as the line is spoken.

Add a line as a title on iPhone or iPad

Open the project and tap a clip in the timeline.
Tap Titles, choose a style, and drag the sample title in the viewer to where you want it.
Tap the title, tap Edit, type the line and tap Done.

For a whole video, burn the subtitles in instead

Timing dozens of titles by hand is slow. Export the edited movie from iMovie, then add the subtitles to that file: generate them from the speech, check the wording, pick a style and export an MP4 with the subtitles burned in.

Things that trip people up

Apple doesn't state that SRT import is unsupported; its guides simply never mention subtitle files, and Apple Community answers confirm iMovie can't import them.
Some theme and animated titles don't let you change the font.
On Mac, a title dropped onto the first or last third of a clip shrinks to that third.
Titles are burned in: viewers can't turn them off, and there is no separate subtitle file.

Questions

Can iMovie import SRT files?

No. Apple's iMovie guides only cover titles, and there is no option to import a subtitle file.

Does iMovie have automatic captions?

No. iMovie has no speech-to-text; every caption is a title you type.

How do I add subtitles to an iMovie video quickly?

Export the movie, then generate subtitles from its audio with a subtitle tool and export an MP4 with them burned in.