The South Australian’s next race was the September world road championship, where he spectacularly defended his time trial crown.
Dennis competed at the worlds on a blacked-out bike.
Days after that win, Bahrain-Merida announced they had released Dennis from the final year of his contract.
Ineos is the team of Tour de France winners Chris Froome, Geraint Thomas and reigning champion Egan Bernal.
Along with retired star Brad Wiggins, Ineos have won all but one of the Tours since 2011.
“Watching on from afar, I’ve always been a huge fan of the team – ever since the early years” Dennis said in an Ineos statement.
“It’s always been a goal of mine to ride for this team, so to finally fulfil that dream is a brilliant feeling and a huge honour.
“I already feel a connection to this team – a team that really believes in innovation, especially in time trialling, which is obviously something I’m hugely passionate about.”
Dennis will join the team at a training camp this month and will start his 2020 campaign next month in Australia.
Ineos did not indicate which Australian races Dennis would enter, but one of them most likely will be Adelaide’s Tour Down Under.
“Racing in Australia is always special, especially for my family who rarely get to see me race – so it’ll be extra special to race for the first time for Ineos in front of them,” he said.