The Jumbo-Visma star was tucked on the wheel of Colombian Fernando Gaviria in the run-in to the line and timed his kick to perfection.
Ewan came from a long way back but ran out of road as Gaviria held on for third place.
“The final straight was really long, so today it was all about the right timing. I had high speed and so when I saw the 200m sign and maybe a bit before, I thought not to wait and I launched my sprint. I had the speed to maintain it to the finish. I’m really happy,” Van Aert said.
Pressed on his GC aspirations, Van Aert added, “For sure I think today and at Strade Bianche we saw a strong [Jumbo-Visma] team.
“We’re in high spirits and big motivation for this week. Starting with a win is perfect for the GC. We’ll see what the next stages bring but we’ve already achieved one goal today.”
Thursday’s stage two will see the riders cover 226km from Camaiore to Chiusdino.
Tirreno-Adriatico stage one results:
1, Wout van Aert, BEL, Jumbo-Visma 3:36:17
2, Caleb Ewan, AUS, Lotto Soudal
3, Fernando Gaviria, COL, UAE Team Emirates
4, Andrea Vendrame, ITA, AG2R Citroen
5, Luka Mezgec, SLO, Team BikeExchange
6, Tim Merlier, BEL, Alpecin-Fenix
7, Alvaro Hodeg, COL, Deceuninck-QuickStep
8, Davide Ballerini, ITA, Deceuninck-QuickStep
9, Ivan Garcia Cortina, ESP, Movistar Team
10, Hugo Hofstetter, FRA, Israel Start-Up Nation