We get this question a lot. The quick answer is that no, at the moment Prismic does not have a write API. This is something that we hope to add in the future, but for now these features might serve your needs.
Migrating Existing Content
If you're looking to migrate existing content from another system into Prismic, you should check out our Import/Export feature.
Integrating with content in external systems
If you have content that lives in another system or in a custom API, it is possible to integrate this with Prismic using our Integration Fields feature.
Have another use-case?
Let us know in the chat support on the Prismic website to help us better understand how we might address this issue with a write API or otherwise.