"Sadly, too many local schools, hospitals and post offices are under pressure and many face closure," said party leader Nick Clegg. "We want to protect the countryside and make it a place people can afford to live and work in, with access to reliable local services."
If elected, the party plans to split the Post Office from Royal Mail, allowing each to "focus on their separate challenges".
The Post Office would remain in public ownership to provide a "lifeline" for all communities, the party said. It also plans to develop a Post Bank and offer government services through the network.
The Lib Dem leader said he believed the Post Office network was an essential means of communication for the British public.
"No other network offers the same level of access as the Post Office: 93.3% of people are within a mile of a post office while only 70% of households have access to the internet," Clegg added.