The Sun on Twitter passed along word of the deal:
The BBC's Oluwashina Okeleji provided a look at Ba's reveal:
Squawka Football on Twitter reveals what number Ba will wear next season:
Ba joined Chelsea last January as the club was looking to bolster attacking options behind Fernando Torres. After successful stints with West Ham United and Newcastle, he seemed like an option that would finally provide the Blues with some stability.
The Senegal international started well, scoring two goals in his debut match. He continued to showcase good form during the early stages of his Blues career, but eventually, he faded and once again left the side with a void atop the formation.
Chelsea even brought in veteran Samuel Eto'o in an attempt to catch lightning in a bottle. His success was also relatively limited.
Since there was depth but a lack of goalscoring from the forward group, the move doesn't come as a major surprise. Ba wasn't able to maintain the early scoring pace he set, and heading to a club where he is likely to find more playing time should help him rediscover top form.
He was linked to Arsenal back in December but said at the time there was nothing going on with the Gunners, noting that things were OK for him, according to Kevin Shield of The Sun (subscription required):
People talk a lot about me joining Arsenal but there has never been an approach from them. There is nothing going on in this regard. Things are OK for me at the moment. It's working as the situation is turning around for me bit by bit.
Then in May, George Scott of the Daily Star provided comments from Ba, who was convinced he was staying at Stamford Bridge for another season:
Jose Mourinho has asked me to stay at Chelsea next season. He assured me that my situation would change.
I see this as my reward from God. I weighed up the pros and cons, and think that I will stay put to see what happens.
Clearly something changed over the past couple months to alter that outlook for both Mourinho and Ba. Only time will tell if it's for the better as the sides go their separate ways.
The 29-year-old striker combines good pace and a nice finishing touch with a willingness to play physical in order to open space for himself in the final third. It worked well with West Ham and Newcastle, but he couldn't match that success with Chelsea.
It wouldn't be a shock to see Ba return to his previous scoring levels with his new club. For whatever reason, Chelsea just haven't had much luck with their strikers over the past few seasons. It's a trend they hope will soon change.
The Blues have been able to remain in the Premier League title hunt because of tremendous play in the midfield and solid defending. If they are able to find a more consistent striker, or simply get better play from those still on the roster, they will be in great shape.
Ba is on the move because it turned out he wasn't the answer.