Police used the gas after several dozen protesters hurled stones at a police bus and stole helmets and shields, an AFP photographer said. At least one demonstrator was injured.
About 1,000 people massed outside the hospital over the death of 15-year-old Berkin Elvan, who was walking to buy bread when he was hit in the head by a tear gas canister fired by police as mass protests swept Istanbul in June.
Elvan's story - he spent 269 days in a coma - gripped the nation and became a symbol of the heavy-handed tactics used by police to reign in the biggest demonstrations against Prime Minister Recep Tayyip Erdogan since he came to power in 2003.
His mother Gulsum Elvan, speaking to reporters outside the hospital, challenged Erdogan who had praised police "heroism" during the protests.
"It's not God who took my son away but prime minister Erdogan," she said through tears.
The June protests began as a small environmental movement to save an Istanbul park and snowballed into a nationwide wave of protests against Erdogan, who critics say has become increasingly authoritarian.
Elvan's death brought the toll from the unrest to at least eight including a policeman.