The former U.S. college player played for the University of Kentucky, and broke the news to his team mates on Wednesday in a video posted on Melbourne United's social media.
"A few years ago I fell into a very dark place, a very lonely place," he tells team mates in the video.
"I couldn't be who I am and I attempted to take my life.
"The main reason behind me becoming so low and being at that point was because I was very much struggling with my sexuality and coming to terms with the fact that I'm gay."
Melbourne United Chief Executive Officer Nick Truelson said the whole club was proud of Humphries.
"Today is an incredible step in Isaac's journey, and we as a club are wholeheartedly supportive of him," said Truelson.
"Not only as a member of our club, but as a person. This is a momentous occasion for him personally, but also for male sport on a global scale."
Very few male players in professional basketball have come out as gay.
In 2013, Jason Collins became the first active NBA player to come out but no other players in the American league have followed his lead.
Pride in Sport, an Australian non-profit advocating inclusion, applauded Humphries but said his announcement highlighted athletes' fears that they might face discrimination, harassment, bullying and "loss of professional opportunities" if they came out.
Adelaide United midfielder Josh Cavallo became Australia's first player to come out as gay in the male top-flight A-League soccer competition last year.
