The men were flying from London Luton to the Slovak capital Bratislava.
The groom was among those detained for endangering security when the flight landed at Berlin's Schoenefeld airport.
They were later released, but could face fines of up to â‚¬25,000 (Â£20,000) and a possible civil damages suit.
The group of 12 from Southampton were among 170 passengers on the Ryanair flight on Friday evening.
As six of the party, including the groom, became drunk, they ignored crew instructions, a Berlin police statement said.
When refused further alcohol, they became aggressive and one member of the group undressed, according to German press reports.
The pilot decided to make an unscheduled landing at Berlin's Schoenefeld airport, where the six men, aged between 25 and 28, were taken into police custody, but later released.
The remaining 164 passengers, including the six members of the stag party who were not involved in the incident, were able to continue their journey to Bratislava.
Six British members of a stag party on a Ryanair flight were arrested for allegedly drunken behaviour which forced an unscheduled landing in Berlin, police there say.