City have responded quickly to losing their Premier League title to Liverpool, bringing in Ferran Torres from Valencia in a £21million deal on Tuesday and now spending £40m on Ake.
A centre-back was Guardiola’s priority during this window after failing to replace Vincent Kompany last summer.
He could yet strengthen further in that department, with Kalidou Koulibaly and Pau Torres among the other players on his radar.
Guardiola told sporting director Txiki Begiristain he wanted to be active in the market in response to Liverpool’s runaway title success.
David Silva will leave at the end of the season, while Leroy Sane has already been sold to Bayern Munich.
John Stones and Nicolas Otamendi also face uncertain futures after slipping down the pecking order this season.
City struggled to cope in the absence of Kompany, while Aymeric Laporte missed the majority of the campaign with a serious knee injury.
It left Guardiola determined to secure top-class options in the centre of defence, which he hopes Ake will provide.