The champions play Manchester United at Old Trafford on Saturday, with the two managers having clashed in the past.
Mourinho called Ranieri "a loser" after replacing him at Chelsea and also insulted him while being the Inter Milan boss.
Ranieri said: "Mourinho is a fantastic manager, intelligent, clever. [The issues] were a long time ago”.
Mourinho replaced Ranieri at Chelsea in 2004 and said the club considered the Italian to be "a loser", before the pair repeatedly fell out while both managing in Serie A.