Chelsea are expected to spend over £400,000 when performance-related bonuses are considered, surpassing the sum paid by Barcelona to sign England international Keira Walsh from Manchester City in 2022.
Ramirez, who has 30 senior international caps for Colombia, said: "I am very honoured to be a part of Chelsea. To be part of such a huge club is a dream come true and one I’ve had since I was a little girl".
Chelsea general manager Paul Green added: "Mayra is a dynamic forward who can play anywhere across the front line. We have watched her develop in Spain over the last few years and believe her style is going to suit playing in England.
"She has great physicality, athleticism and a good goal scoring record. We’re all excited with what she can bring to the squad now and in the long-term."
Chelsea have also agreed a deal in principle to extend Kerr's contract, as it was due to expire at the end of the season.
The Australian superstar, who has scored 99 goals in 128 games for Chelsea, remains out for the season having had surgery on a ruptured knee ligament, but her long-term future is secured.
Chelsea are keen to give Hayes a sensational send-off while positioning the club to withstand her exit after 12 glorious years in west London.
The Blues are top of the Women's Super League, remain in both domestic cup competitions and are through to the knockout stages of the Champions League.