BMC dominated Quick-Step Floors and Team Sunweb with a time of 15min 58sec over the 13.7km city course.
Chris Froome and Team Sky took the fourth place ahead of most of their rivals for the general classification.
BMC won the ITT in 2015 and Dennis is the first Australian rider to wear the red jersey since Michael Matthews kept it for three days in 2014.
SBS will broadcast and live stream all stages of the Vuelta a España live from 19 August to 10 September.
Lotto-Soudal set the first reference time of 16min 22sec before BORA-Hansgrohe bettered that benchmark. Sunweb was next in the hot seat with a time of 16 min 4sec after half the teams had finished their runs.
Quick-Step Floors then took the lead with a time only a hundredths of a second better than Sunweb, before BMC managed to build on that with an average speed of 51.5km/h giving them a time of 15min 58sec.
In the pending battle for the general classification, Chris Froome (Sky) finished ahead of most of his potential rivals. Esteban Chaves and the Yates brothers (Orica-Scott) are eight seconds in arrears, Vincenzo Nibali (Bahrain-Merida) 22 seconds, Alberto Contador (Trek-Segafred) 26, and Fabio Aru (Astana) 32.
Dennis will wear the red jersey in the flat 203.4 km second stage, which also takes place in the southern French city before moving into Spain for stage four on Tuesday.