Jose Mourinho hinted Chelsea had given up their pursuit of Manchester United striker Wayne Rooney after two rejected bids and now has turned to 'plan b'.
Mourinho and the Cameroon striker are to be reunited after working together at Inter Milan, with Chelsea and Russian club Anzhi Makhachkala having agreed a deal.
Eto'o said on chelseafc.com: "I am very happy to be here but also anxious because I want to get playing as soon as possible.
"It wasn't a hard decision. I saw the qualities Chelsea have, and I was very happy with Jose Mourinho before, so when the opportunity came, I was very happy to take it."
Capturing Eto'o, who commanded wages reported to be in the region of £300,000 a week in Russia, is at odds with Chelsea's recent transfer policy of signing younger players.
It also leaves a question mark over the futures of Demba Ba and Fernando Torres as Mourinho has made it clear four strikers is too many and that Romelu Lukaku is key for Chelsea's future.
Ba's place in the squad seems most at risk, but Torres has still to repay the £50million fee paid to Liverpool, as Mourinho admitted earlier this week.
Eto'o is the second player to sign from Anzhi in a matter of days, after Willian's capture was announced on Sunday and completed on Wednesday. Winger Willian did not travel to Prague with the squad for Friday night's European Super Cup clash with Bayern Munich, but can be expected to provide the ammunition for prolific striker Eto'o.
Speaking prior to confirmation of Eto'o's arrival, Mourinho appeared to have conceded defeat in the bid to capture England striker Rooney, who played for United against Chelsea at Old Trafford on Monday, after hinting a third bid had not been lodged.
Mourinho said: "I told you what I have to say. I think I (made it) very, very clear that we didn't start the process (a third bid).
"We have nothing more to say. He's a Man United player and no more than that."
The 32-year-old Eto'o began his career with Real Madrid before, following a spell with Real Mallorca, surging to prominence with Barcelona, scoring 152 goals in 232 appearances for the Catalan club before moving to Inter as part of the deal which took Zlatan Ibrahimovic in the opposite direction.
Eto'o has a controversial history with Chelsea, from his time playing for Barcelona.
Following a bad tempered Champions League encounter at Stamford Bridge, the striker criticised Mourinho, referee Pierluigi Collina and made a claim in the Spanish media that he was called a monkey by a Chelsea security guard. The allegations were swiftly denied by Chelsea.
It can be assumed Eto'o's relationship with Mourinho did not suffer as at Inter the pair combined to devastating effect and together won the 2010 Champions League title in a treble-winning campaign.
The striker netted 53 goals in 102 appearances for Inter before joining Anzhi in August 2011.