Working fathers are paid a fifth more than childless men in similar jobs, according to a new study.
Research by the TUC found a wage "bonus" of 21%, falling to 9% for men with two children.
The study was in "stark contrast" to a pay gap found between working mothers and those without children.
The reason for fathers earning more may be due to longer working hours and putting in more effort, said the report.
Employers may view fathers as being more reliable, while CVs from mothers were marked down, the TUC said.
General secretary Frances O'Grady said: "In stark contrast to the experience of working mums, who often see their earnings fall after having children, fatherhood has a positive impact on men's earnings.
"It says much about current attitudes that men with children are seen as more committed by employers, while mothers are still often treated as liabilities.
"While men play a much more active role in raising their children nowadays, many are afraid to request flexible working or time off in case it damages their career prospects.
"We won't break this cycle unless fathers are given access to independent paid leave to look after their kids, that isn't shared with their partners.
"We need more decently paid jobs to be available on a reduced hours or flexible work basis. This would reduce the motherhood pay penalty and enable more dads to take work that fits with their parenting responsibilities."