Currently, there is no public VALORANT API.
No ETA just yet. We're working on it. Want to have it ASAP. My hope is that we're in the weeks range, but it's too early still to commit to anything. Once we can commit, you'll likely see a post about upcoming VALORANT apis and what they'll look like.
Riot has said publicly that they working on the API but no specifics. They plan to launch the API during the "launch/kickoff period" they have said "around 2 weeks after release."
In other words, soon™. June 16th is that 2 week mark, but don't hold your breath.
However, Riot Games has opened up API applications to VALORANT. You can submit your request at developer.riotgames.com. No documentation or official features have been announced yet, but Riot Gene has been actively promoting the features on Twitter and Reddit.
We hope to provide robust APIs for VALORANT similar to the the LoL/TFT/LoR APIs where people will be able pull data in the future.
This is nothing but good news for players, streamers, pros, and fans. With Riot's commitment to robust statistic APIs, it is exciting to think about what the community will come up with. Following kill/death ratio per character, win rate by map, round, agent, accuracy, headshot accuracy, play time -- The options are virtually endless.
We have enabled submissions for @PlayVALORANT products on http://developer.riotgames.com.
If you are interested in developing for VALORANT, please submit your ideas/products and I'll get back to you.
We at VALORANT Haven will be keeping a close eye on this. We have applied to get access to the API and plan to take full advantage of it as soon as we possibly can.