— British Cycling (@BritishCycling) June 23, 2016
It's the Essex rider's fifth national TT title, having won it between 2011-13 and 2015 - Bradley Wiggins won it in 2014.
Dowsett clocked 55-38.523 on the 46.4km (28.8 miles) course to beat second-placed James Gullen (Pedal Heaven) by one minute and 16 seconds.
Ryan Perry (Langdale Lightweights) placed third.
There were a few notable non-starters in the event, which reduced the field to just 34 riders. Last year's runner-up and pre-race favourite for another medal, Ed Bradbury was absent, as were Team GB track pursuiters Owain Doull and Steven Burke. George Atkins, Mark Christian and Dan McLay were also listed as non-starters.
Dowsett is currently on Movistar's long list for the Tour de France, where he is hoping to be picked for the final nine riders and contest the race's time trial stages in the jersey of British national champion.
Earlier in the day, Scott Davies (Team Wiggins) took the under-23 men's title and Hayley Simmonds (Aerocoach) defended her elite women's crown.
The British road national championships continue on Sunday June 26 with the road race events.
British time trial national championships 2016: Elite men
1. Alex Dowsett (Movistar) in 55-38.523
2. James Gullen (Pedal Heaven) in 56-54.83
3. Ryan Perry (Langdale Lightweights) in 57-52.53
4. Peter Williams (One Pro Cycling) in 58-15.36
5. Richard Handley (One Pro Cycling) in 59-09.39
6. Samuel Harrison (Team Wiggins) in 59-11.87
7. Lloyd Chapman (Pedal Heaven) in 59-36.73
8. Ashley Cox (CC Luton) in 59-45.70
9. Andrew Tennant (Team Wiggins) in 59-59.87
10. Rhys Howells (Richardsons-Trek) in 1-00-4.48
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