All 30 riders on the Team Ineos roster - split across six teams - took part in the 30km race on virtual cycling platform Zwift, including Tour de France champions Chris Froome, Egan Bernal and Geraint Thomas.
Dennis attacked early on the final climb, hitting the finish line atop Alpe du Zwift just over 56.11s ahead of Eddie Dunbar with Michal Kwiatkowski rounding out the podium (at 59.20s).
Cameron Wurf, who stuck with fellow Aussie Dennis on part of the final climb, crossed the line fourth (at 2:03) with Chris Froome the only Ineos grand tour champion to finish in the top 10 (sixth: 02:33)
“There was supposed to be a plan," Dennis said.
"But in the end I felt it was better to just send it, why not? It worked out!
“It was a good little hit out, I spotted Cam (Wurf) with me and I couldn’t let him sit on my wheel for too long, so I pushed a bit harder and dropped him, then I could sit on my TT pace from there.
“It’s a time when nobody is allowed to ride with anybody so we’re lucky to have technology like this so we can do something for the fans to keep them across what we’re doing and how we’re training.”
Over 100,000 people tuned in to either the livestream or the replay of the livestream on YouTube with around 15,500 people joining the team on an earlier mass participation eRide, breaking Zwift's single event record.
"Fair play to Rohan," Team Ineos principal Dave Brailsford said. "You can see why he’s the world time trial champion.
"You can see Eddie is in pretty good shape, but I think the ride that caught my eye was Chris Froome.
"That’s a hell of a ride from a guy who is coming back and that’s a great indicator that he’s right back on track and going to be in the mix when we get back out there on the road."