Males fight for access to females by using their teeth and flippers. Mating occurs on the ice and the seals are monogamous for the season.
Sexual maturity is reached in males at 6 years and in females at 5 years. The reproductive season for harp seals is from the end of February to the end of March. The seals migrate south to breed and molt. Gestation lasts 11.5 months, with 4.5 months dormancy of the fertilized ovum (delayed implantation). Females give birth to one pup, occasionally to twins. The pups weigh about 10 kg and are suckled for about 12 days. The milk is rich in fat and mostly becomes blubber. Pups can gain up to 2 kg a day. During lactation, the mother feeds on little if anything at all. After weaning, the young spend up to 10 days alone until their embryonic hair is replaced by proper fur.