The result has moved the Cherries into eighth spot in the Premier League table, just three points behind fourth-placed Arsenal, while Man City have been knocked off the summit, with Pep Guardiola's side now in second spot behind Liverpool.
The defeat is Man City's first in the Premier League since last December, when Aston Villa recorded a 1-0 win at Villa Park, while they have been beaten by Bournemouth for the first time in history.
Bournemouth had a big chance in the early moments at the Vitality Stadium, with a strike from the dangerous Semenyo being parried by Man City goalkeeper Ederson into a dangerous area, but the Brazilian then managed to keep out the rebound from Justin Kluivert.
The Cherries did take the lead in the ninth minute, though, with Semenyo turning and finding the back of the net with a clever finish after smart work from Milos Kerkez in a wide position.
Man City had five attempts in the first half but failed to hit the target, with Bournemouth relatively comfortable in the opening 45 minutes against the champions, who were finding it difficult to get going in the tricky away fixture.
Kerkez was at it again in the 64th minute, with the impressive left-back driving down the left before delivering a brilliant low cross into Evanilson, and the striker was on hand to turn into the bottom corner of the net.
The Citizens halved the deficit in the 82nd minute courtesy of a header form Josko Gvardiol, but Bournemouth survived a host of late pressure to record a famous victory over the reigning English champions.
BOURNEMOUTH VS. MAN CITY HIGHLIGHTS
12th min: Bournemouth 1-0 Man City (Antoine Semenyo)
Bournemouth make the breakthrough in the ninth minute of the match, as Kerkez does brilliantly down the left before delivering a cross into Semenyo, and the in-form attacker manages to turn and find the back of the net with an excellent finish.
Semenyo has now scored four Premier League goals this season.
64th min: Bournemouth 2-0 Man City (Evanilson)
Bournemouth double their lead against the champions, as Evanilson turns a cross from Kerkez into the back of the net; it is a wonderful team goal from the Cherries, who now have a two-goal advantage on home soil.
82nd min: Bournemouth 2-1 Man City (Josko Gvardiol)
Man City have a goal back here, as Gvardiol brilliantly finds the back of the net with a header; the cross came from Ilkay Gundogan, and the goalscoring defender was on hand to power his effort past Mark Travers.
MAN OF THE MATCH - MILOS KERKEZ
Kerkez enjoyed an excellent first season with Bournemouth in 2023-24, and the left-back has again been impressive this term, with a number of major clubs, including Manchester United, thought to be keen on his signature.
The 20-year-old was brilliant on Saturday afternoon, coming up with assists for both of Bournemouth's goals, while he was defensively brilliant, particularly in the second period, as Man City were shocked at the Vitality Stadium.
BOURNEMOUTH VS. MAN CITY MATCH STATS
Possession: Bournemouth 35%-65% Man City
Shots: Bournemouth 12-19 Man City
Shots on target: Bournemouth 6-4 Man City
Corners: Bournemouth 3-10 Man City
Fouls: Bournemouth 10-6 Man City
BEST STATS
WHAT NEXT?
Man City will be bidding to avoid a third straight loss in all competitions when they head to Sporting Lisbon in the Champions League on Tuesday night, before travelling to Brighton & Hove Albion in the Premier League next Saturday.
Bournemouth, meanwhile, will be aiming to extend their unbeaten run in the Premier League to four matches when they head to Brentford on the same afternoon.