In Mad Men, Pete Campbell is unaware that he has fathered a child with Peggy Olson.
Pete and Peggy have sex early in the series, including after Pete's bachelor party and again in his office. Peggy becomes pregnant but conceals the pregnancy from her co-workers and gives birth to a son, whom she places for adoption. Pete does not initially know that the baby is his.
The secret becomes especially painful because Pete is simultaneously trying to build a conventional marriage with Trudy Vogel. In the second-season finale, during the Cuban Missile Crisis, Pete tells Peggy that he loves her and wishes he had married her. Peggy then reveals that she had his child and gave the baby up for adoption, leaving Pete stunned.
This plot is sometimes confused with the later revelation that Pete's wife Trudy is pregnant. That is a separate pregnancy, and Pete is initially unaware of it as well, but the question specifically concerns Peggy's child. Pete Campbell is therefore the single canonical answer.