If a WebM file arrives on an iPhone through Messages, Mail, AirDrop or WhatsApp, iOS generally treats it as an unrecognised document. There is no thumbnail and no way to play it from the Files app. This is not a fault with the file or with your phone.
What Apple actually supports
Apple built its entire media stack around H.264 and, more recently, HEVC. Both have dedicated hardware decoders in every iPhone chip, which is how a phone plays 4K video for hours without getting hot. VP8 and VP9, the codecs inside WebM, have no such hardware support on Apple silicon.
Safari gained limited VP9 playback in Safari 14 for streaming purposes, which is why some WebM videos play on web pages. But that support is confined to the browser engine. The Photos app, the Files app, QuickTime, AirPlay and the share sheet do not use it, so a WebM saved to your phone remains unopenable.
What you can do on the phone itself
- Install VLC for iOS. It is free, has no ads, and decodes WebM in software. Save the file to Files, then open it in VLC through the share sheet. This is the quickest fix if you only need to watch it once.
- Open it in a browser. If the video is hosted online rather than sent as an attachment, Chrome for iOS or Safari may play it directly on the page even though the downloaded file will not open.
- Convert it to MP4. The only option that produces a file which behaves normally — appearing in Photos, playing on tap, AirDropping, and uploading to anything.
Conversion works from mobile Safari or Chrome as well as on a desktop, since the encoder runs inside the browser. Expect it to be slower on a phone, and keep clips short — a few hundred megabytes is a realistic ceiling for a phone browser tab.
Sending video to iPhone users
If you are the one distributing the file, convert to MP4 before sending. It costs you a minute and saves the recipient from a problem they may not know how to describe, let alone solve. This applies to anything sent through Messages, email, Slack, WhatsApp or a shared drive when Apple devices are in the mix.
Why Apple has not simply added support
Adding a software decoder would work but would drain battery and heat the device, undercutting the efficiency argument that Apple's hardware decoding is built on. Adding hardware support means allocating silicon to a competing format at a time when the industry is moving toward AV1 anyway. Recent Apple chips have started including AV1 decoding, which suggests VP9 will remain a gap rather than ever being filled.