The 1,400-seat playhouse, just a short distance from the old port, is a masterpiece of early 20th-century Spanish architecture in the once-international city that in its heyday hosted a wealth of colourful characters and communities.
But today the blue and yellow plasterwork and magnificent frescoes on the ceiling are crumbling, the dust-covered seats unoccupied since 1974, the year the curtains fell for the last time.
"You have to imagine how Tangiers was at the time. A census in 1919 registered a population of 40,000, including 26,000 Muslims, 5,000 Jews and 6-7,000 Spaniards," says Bernabe Lopez Garcia.
Last month, the Spanish historian organised an exhibition to celebrate the theatre's centenary.
But it was also a plea for help in restoring this cultural treasure, now more of a wreck at the end of a sidestreet, near to but also a world away from the new luxury marina being built on the city's Mediterranean seafront.
"It's like a theatre of shadows.... You make me come back, but I do so reluctantly, because each time I feel bad," says Rachid Taferssiti, president of Al Boughaz, an association dedicated to preserving the city's heritage.
"I find it sad that a multicultural space like this is allowed to fall into such a state."
The building was commissioned in 1911 by a rich Spanish merchant, Manuel Pena, who dedicated it to his wife Esperanza Orellana, a passionate theatre-goer.
Inaugurated in 1913, its history charted the Spanish presence in Tangiers, a city clearly visible from and sharing a tangled history with Spain.
During World War II, the city was occupied by Spain from 1940 until 1945. Spanish forces in the city disliked the building's modernist appearance, and planned "to convert it into an imperial theatre in the neo-classical fascist style", says Lopez Garcia.
The theatre's activity peaked in the 1950s, when the Spanish community grew to around 30,000.
But the city on the crossroads between Europe and Africa, the Atlantic and Mediterranean has long captivated the imagination of foreigners and attracted a diverse mix of visitors.
It was something of a Mecca for French painters in the 19th century, while a number of celebrated US writers settled in Tangiers after its designation as an international zone in 1923, notably Paul Bowles and William Boroughs.
Artists who performed at the playhouse include legendary Italian tenor Enrico Caruso and Spanish-born opera star Adelina Patti, as well as Flamenco troupes and Arabic ensembles.
"The theatre group Al Hilal, composed of local Moroccan actors, played Othello in 1929," says Rachid Taferssiti, whose own father performed in various plays.
Since it closed, the Gran Teatro Cervantes has been loaned to Morocco for a symbolic dirham per year ($0.12, $0.09), while remaining Spanish state property, with the neighbouring countries unable to agree on a renovation plan.
"I think the Spanish government would like nothing better than to be able to restore it. But with the current crisis, it's impossible for them to address the issue," says Cecilia Fernandez Suzor, the director of Tangiers' Cervantes Institute, a Spanish cultural body.
She admits the building is in a "pitiful state", adding: "If you want to redo it as it was in 1913, show us your millions!"
On top of the cost itself, estimated at four million to five million euros, the theatre's location in a neglected corner of the city remains another obstacle.
"But you often find examples of a restored cultural site leading to the transformation of the area around it," Fernandez Suzor says optimistically.
She suggests the theatre could also serve as a training centre for stage-related professions.
"We have our role to play, but the solution lies with the Spanish and Moroccan governments," says Taferssiti, pointing to the huge changes in the form of government-funded projects taking place outside Tangiers.
Indeed, the authorities rarely waste an opportunity to publicise the planned economic transformation of Morocco's second industrial city, driven by a vast industrial development programme launched in September and expected to cost $1 billion.