Amazon's games streaming platform Twitch has lost one of its biggest stars to a rival site owned by Microsoft.
Tyler Blevins, known to fans as Ninja, had 14.7 million followers on Twitch and mostly played Fortnite.
Mixer was launched in 2016, but has lagged behind the popularity of Twitch and YouTube Gaming.
In a statement, Microsoft said it was "thrilled" to bring Mr Blevins on board.
"Mixer is a place that was formed around being positive and welcoming from day one, and we look forward to the energy Ninja and his community will bring."
Mr Blevins, 28, told his fans via Twitter about the move.
"I have been holding on to this for quite some time," he said.
"I'm just super-excited to let everyone know. I'll be streaming on Mixer full-time now and I'm honestly at a loss for words. I'm freaking out in the best ways. I feel like I'm going to get back to the streaming roots."
The financial terms of the deal have not been made public. As well as streaming exclusively on the platform, Mr Blevins will also represent and promote Mixer at events.
Divided fan base
In a peak month, Mr Blevins was making a reported $500,000 streaming on Twitch. The move is likely to divide his fan base, Variety magazine's Janko Roettgers said.
"Ninja has a dedicated fan base, and many will likely tune in no matter the platform," he said.
"At the same time, the departure is also a chance for other Twitch streamers to scoop up some of the more casual viewers of Ninja's streams."
At the time of writing, 80,000 people had subscribed to Mr Blevins's new channel on Mixer.
Mr Roettgers said the signing, probably for a huge amount of money, was a clear signal of intent from Microsoft to catch up in the streaming sector.
"Microsoft is trying to find new gaming revenue streams that go beyond selling hardware," he said.
In a statement following news of Mr Blevin's departure, Amazon said: "We've loved watching Ninja on Twitch over the years and are proud of all that he's accomplished for himself and his family and the gaming community.
"We wish him the best of luck in his future endeavours."