Stitch has released a native iOS app which is available on the Apple App Store. Check it out at the link here: https://itunes.apple.com/us/app/stitch-companionship-dating/id961684150
Alternatively, if you search on your phone in the Apple App Store for 'Stitch companionship' you will be able to find and install the app.
NOTE: Because of Apple's requirements, the iOS app does not support verification or the ability to choose a membership plan. These must be done in a web browser, either on your phone, tablet, or a desktop computer. Apart from that, the Stitch app provides access to all of the same features available on the desktop site.
Stitch is designed to work on all devices, no matter how big or how small. You can expect the same simple, safe and easy-to-use experience whether you’ve got an iPhone, iPad, Android phone or are just accessing Stitch via your browser on a Mac or PC.
At this stage Stitch doesn't have an Android app but we hope to create one some time soon.