The Bora-Hansgrohe rider just avoided French rider Olivier Le Gac crashing in front of him during the finale before outsprinting Italian champion Elia Viviani and Australian Caled Ewan.
Slovenian Primoz Roglic retained the overall leader's pink jersey after remaining in the peloton throughout a rainy day after the race left Emilia-Romagna for Tuscany.
Eight men broke away early on but the bunch always had them under control, reining them in shortly before the line to trigger a massive sprint.
"It was our first chance to win a stage and we did it straight," Ackermann, who is making his grand tour debut, told reporters.
"We were super-motivated. It bodes well for the rest of the Giro."
Roglic, who impressed in Saturday's opening time trial, said: "It wasn't an easy day because it was cold from the beginning and also quite fast all the time.
"It will be another day in the 'Maglia Rosa' tomorrow. The team worked hard for that. It's a pleasure to retain the jersey.”
Monday's third stage will take the peloton over 220km from Vinci to Orbetello.
(Reporting by Julien Pretot; Editing by Toby Davis and Tony Lawrence)
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