The props comedian - born Leo Gallagher - began to experience chest pains prior to a scheduled performance on Wednesday evening (March 14). The venue owner called for the emergency services, and the 65-year-old was admitted to an undisclosed hospital.
Gallagher's manager told TMZ that he will remain in hospital for a couple of days under observation.
It was also confirmed that Gallagher has suffered a "mild to serious" heart attack.
Gallagher was admitted to hospital 12 months ago after he developed chest pains on stage during a performance in Minnesota. He was discharged three days later, following what was diagnosed as a mild heart attack.
He started performing in the 1970s, making several appearances on The Tonight Show when regular host Johnny Carson was absent, and later became popular after he began to close his stage act by smashing objects such as watermelons with a sledgehammer.