The Australian, who joined the Hammers as a 14-year-old, made his first team debut in 2012, having already been diagnosed with testicular cancer.
The problem was discovered in 2011 when Tombides underwent a random drugs test after playing for the Australia U-17 team at the U-17 World Cup.
Despite his diagnosis, the Perth native continued to train with the Hammers and made his senior debut as a replacement in a League Cup match against Wigan in 2012.
“West Ham United announce the sad news that forward Dylan Tombides has passed away at the age of 20 following his brave battle with cancer,” the club said in a statement.
“Dylan passed away on Friday morning with his family by his side having courageously fought the disease for three years after initially being diagnosed with testicular cancer in summer 2011.
“Dylan’s amazing resilience and positivity saw him through months of surgery and chemotherapy, while his outstanding talent saw him make his first-team debut in a League Cup tie with Wigan Athletic at the Boleyn Ground in September 2012.
“Away from the pitch, Dylan did a huge amount of work to raise awareness of male cancer, supporting the One for the Boys campaign at a number of high-profile events alongside the likes of Hollywood star Samuel L Jackson, snooker star Jimmy White and fellow Australian Peter Andre.
“Dylan was respected by everyone who knew him for his intelligent views on the game and his larger than life character. He was a loving son, amazing brother and well-respected member of the West Ham squad. He will be hugely missed by everyone who had the honour of knowing him.”
A minute’s silence will be held in Tombides’ memory before West Ham’s clash with Crystal Palace and the Hammers players will wear black armbands.