The man, who suffers from mental problems, was discovered naked in a tiny room without a roof in the southern town of Dos Hermanas, on a filthy mattress next to which bottles and buckets were laid out for him to go to the toilet, police said in a statement.
Police in the town near Seville made the grisly discovery when they brought his 76-year-old drunk brother back to his home following complaints of disorderly behaviour.
"Once inside the home, they saw a door at the end of a corridor closed with a chain... and padlock," Spain's national police said in a statement.
"After asking the man, he said that was where his brother lived, who had mental problems and whom they kept isolated for his security."
His 61-year-old sister also lived in the house and said she kept him fed and cleaned, and looked after his €1,000 ($1,100) monthly pension.
"Due to the strange situation they were faced with, the agents asked both siblings to open the... door to check the state of their brother," police said.
"Once they went through, they came across another door closed by a wooden beam."
"After going through both doors, they found a crumbling, narrow staircase that went up to a type of dovecote," they added, before discovering the brother in the tiny roofless hovel.
"His health and cleanliness were deplorable and the state of the place was completely insanitary... without access to running water or toilets," police said, describing his living conditions as "inhuman."
The man was taken to hospital, while his two siblings were detained, they added.