Work completed so far includes upgrades to the Royal Circle toilets, enhanced electrics, Wi-Fi and water in the building.
The Theatre's main stage has seen no shows since mid-January whilst upgrades to the stalls seating, repainting of the auditorium and foyers have been going on.
Theatre Manager Josh Brown said:
"We're delighted to reopen our doors to audiences today after being closed for three weeks of internal building works!
"Our stalls seating has been replaced to increase accessible seating options and comfort for all visiting patrons, alongside restoration of the décor throughout the auditorium, stalls and royal circle foyers including the carpets, walls and paintwork.
"We hope our customers will be as excited about these upgrades as we are!"
The Theatre reopens with a jam-packed week of one night only events:
Last night (5 February) saw Adam Kay take to the stage with his new show Undoctored, Today sees Rockinghorse Children's Charity Comedy Night with an incredible line up featuring the fabulous Joe Wilkinson, Katherine Ryan, Maisie Adam, Suzi Ruffell, Jen Brister and Tom Davis.
Thursday see the return of QUEENZ: The Show With Balls! Followed by a return to comedy with Guz Khan on Friday and Rosie Jones on Saturday.