The Cannondale rider sprinted to victory at the end of the 201km stage from De Panne to Koksijde, the third year in a row that he has won the race opener - but he tried to gift his victory to new team-mate Oscar Gatto by slowing down on the line.
He left his braking too late, however, and Gatto lost out in a photo finish. Kenneth van Bilsen of Topsport Vlaanderen came third.
Sagan had attacked 10km from the end of the stage to reel in a seven-man breakaway, with Gatto joining him as he did so.
The race continues with a 206km stage on Wednesday before finishing off on Thursday with two stages, the second of which is a 14.3km time trial.
1. Peter Sagan (Svk) Cannondale
2. Oscar Gatto (Ita) Cannondale
3. Kenneth Van Bilsen (Bel) Topsport Vlaanderen - Baloise
4. Gert Steegmans (Bel) Omega Pharma Quick Step
5. Laurens De Vreese (Bel) Wanty - Groupe Gobert
6. Mauro Finetto (Ita) Neri Sottoli
7. Vincent Jerome (Fra) Team Europcar
8. Niki Terpstra (Ned) Omega Pharma - Quickstep
9. Jérôme Baugnies (Bel) Wanty - Groupe Gobert
10. Sander Cordeel (Bel) Vastgoedservice - Golden Palace Continental Team
- Sports & Recreation
- Oscar Gatto
- Topsport Vlaanderen
- Peter Sagan