Hägglund has led the party for eleven years and sat as Minister of Health and Social Affairs in the previous centre-right alliance government for eight years.
At a press conference today, Hägglund said that leading a party for so long takes its toll, and that he feels like it's time for a change.
There has already been a lot of talk about who will replace Hägglund, even though there is no clear front runner for the position.
The three most commonly mentioned names are party secretary Acko Ankarberg, the party's economic spokesperson Jakob Forssmed and PM Emma Henriksson. All three are close to Hägglund. But even they were surprised by Hägglund's decision.
If the Christian Democrats choose to take an even more conservative path, they may elect Ebba Busch Thor, from Uppsala.
The party board will elect a new leader this spring.