Fathers are currently allowed two weeks but the party wants to go further and increase the protected entitlement to six weeks, extending total parental leave to 58 weeks.
The policy goes further than the Coalition's introduction of shared parental leave from April with the first parents eligible - an estimated 285,000 working couples - learning they are expecting now.
Business and Equalities Minister Jo Swinson said shared paternal leave played an essential part in building a stronger economy and a fairer society in which everyone has the chance to get on in life.
She added: "It allows couples to choose how to split time off work to look after their new baby.
"Extending paternity leave is an important next step to encouraging new dads to spend more time with their child in those vital early weeks and months after birth.
"When parents share caring responsibilities, more equality in the workplace will follow.
"It is a nonsense to think it is only the mother's job to look after children. Parenting is a shared responsibility.
"The 'use it or lose' it six weeks will establish the important role of dads early on, and encourage couples to use the full flexibility on offer."
Under the plans, first suggested in a policy paper last summer, legislation would be amended to provide parental rights to cover six weeks reserved for working fathers, six weeks for working mothers and the remaining time available to share between partners.
In a same-sex couple, each partner would be entitled to six weeks' reserved leave, with the rest available to share.
The Lib Dems say the commitment is necessary due to concerns that, even with shared parental leave, fathers may not take up the opportunity.
According to the party, international evidence suggests fathers' use of parental leave is higher under programmes which offer targeted or reserved leave schemes as opposed to just making shared leave available.
A Lib Dem spokeswoman said she could not confirm at this stage whether or not the policy would be a red line issue in the event of another coalition, but added: "It's very important to us.
"We have done lots in government so far to make sure fathers get more rights. This is just the extra step in encouraging them to spend more time with their children."