The 205km second stage from Bologna came down to a sprint finish in which the German national champion, making his Giro debut this year, had the power to hold off Italy’s national champion.
Australian Caleb Ewan of Lotto-Soudal came in third behind the Deceuninck-QuickStep rider, with UAE Team Emirates’ Fernando Gaviria fourth.
The bunch finish meant there was no change at the top of the general classification, in which Team Jumbo-Visma’s Primoz Roglic holds the pink jersey with a 19-second advantage over Mitchelton-Scott’s Simon Yates.
Two-time Giro winner Vincenzo Nibali of Bahrain-Merida is a further four seconds behind the Lancastrian in third place, with Tao Geoghegan Hart of Team Ineos in eighth place, 35 seconds off pink.
After Saturday’s time trial, Sunday’s stage set off from Bologna in damp conditions, with an eight-man breakaway quickly setting off up the road and building a lead of four-and-a-half minutes.
Bora-Hansgrohe did a huge amount of the work to chase them down on the approach to Fucecchio, with the catch made inside the last 10km.
The team looked to have blown their advantage inside the final 1,000m as others swamped them on the front, but Ackermann kept his cool and launched his attack right on cue.
“I’m so happy, this is my first chance to win a stage and we did it great,” said Ackermann, selected by his team ahead of Irishman Sam Bennett, who won three stages of last year’s Giro.
“I think that all the team is more motivated and it’s good for the next three weeks.”
*** Yorkshire riders finished first and second in the prestigious Lincoln Grand Prix with Sheffield’s Tom Stewart just holding off Leeds rival Tom Pidcock at the end of the tough 103-mile event.
Stewart, a 29-year-old rider for Canyon dhb, was around two seconds clear of 19-year-old Pidcock as he crossed the line on the cobbles of Castle Hill, while his Canyon team-mate Andy Tennant was another few seconds back in third.
It was the second time Stewart has won the prestigious Lincoln Grand Prix. This time his win came after having been part of an earlier break that was pulled back. But he still had enough left in the tank to attack again and take victory.
Yorkshire riders had to settle for the minor placings at the City Road Club Hull 10-mile time trial, with Manchester BC’s Joseph Dobson taking the win.
Hull Thursday Road Club’s Amanda Dean was third in the women’s race, clocking 27.34 to finish behind joint winners Karen Brooks (Team Sportslab) and Diana Bite (Univ of Manchester).