A £7m refurbishment of the Bristol Old Vic will begin in August, with the theatre to be shut until December 2008.
The building will get new seats, new air conditioning, disabled lifts and better stage visibility after the stalls are lifted.
Simon Reade, artistic director at the theatre since 2003, will leave the company in June.
Some redundancies are expected as the theatre said it would reduce its team to "skeleton staff" during the works.
All shows in the summer programme will run, and anyone who had bought tickets for Rough Crossings in September will be contacted.
The refurbishment work will also include structural work on the grade one listed building, and will "address electrical and plumbing issues".
A public appeal will be launched later in May to raise the outstanding £2m to complete the renovation.
Rupert Rhymes, chairman of trustees at the theatre, said: "Of course the decision to close the theatre is a difficult one.
"But at the end of the day we had very little option given the deteriorating state of the building, issues of health and safety and the need to improve the comfort for audiences in the future.
"There is much work to be done and unfortunately this cannot be carried out while the theatre is in operation."
Mr Reade added: "I am honoured to have played a leading role in this latest act of Bristol Old Vic's long history.
"The continual presence of a producing theatre in Bristol since 1766 has made a significant contribution to the culture of the nation as a whole."