The National Association of Head Teachers, representing 28,500 heads, has joined the Trades Union Congress.
The NAHT is the biggest union to join “for some time”, says the TUC, and the first dedicated to school leaders.
TUC general-secretary Frances O’Grady said it was “good to see NAHT joining the TUC’s family of education unions”.
NAHT general-secretary Russell Hobby said he was delighted as “the interests of students, teachers and school leaders are inextricably linked”.
Other education unions already in the TUC include the NUT, NASUWT, the ATL and the Scottish and Welsh education unions EIS and UCAC.
“There is a great deal we can do together to create great places to learn and great places to work,” said Mr Hobby.
The NAHT has members throughout the UK, including head teachers, deputies, assistant heads, bursars and school business managers.
The union says it represents 85% of primary school leaders, 40% of secondary school leaders and “virtually all” the heads in special schools in England, Wales and Northern Ireland.
Welcoming the move Ms O’Grady added: “Education matters, to pupils, to parents, to communities and to the UK economy.”
The Association of School and College Leaders (ASCL), which represents most secondary school heads, remains independent of the TUC.