"Brother Kaczynski, soon you and I and our whole generation will enter into eternity," the 74-year-old Walesa wrote to Kaczynski, who turned 69 and spent several weeks in hospital this year.
"I would like to get my things in order. I would also like to leave reconciled with my foes," added the former president and Nobel Peace laureate who led the Solidarity movement that brought a peaceful end to communism in Poland.
Asked by AFP Thursday for his motives for reaching out, Walesa denied that the move was linked to health problems on either side. He said he was guided by the Christian imperative of reconciliation.
Kaczynski has yet to respond but the spokeswoman for his Law and Justice (PiS) party, Beata Mazurek, tweeted that "we can no longer take Walesa seriously" and refused all other comment.
Walesa ended his message to Kaczynski saying: "If I did you wrong, then I ask for your forgiveness. As for me, I am able to forgive you and your late brother Lech for everything, even the lousiest thing you've ever done to me which was... the Kiszczak folder."
The reference is to old allegations which resurfaced in 2016 that Walesa collaborated with the communist secret police in the early 1970s, something Walesa has long denied.
Although both men fought Poland's communist regime, they later became bitter foes amid power struggles in the early years of Poland's democracy.
More recently Walesa has been highly critical of Kaczynski and the PiS, which won power in October 2015 and has since pushed through a string of overhauls that led to mass protests at home and conflict with the European Union over rule of law violations.
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