The incident happened in Fox Way.
Emergency services, including the air ambulance, were sent to the scene of the collision, which involved two motorcycles - a Suzuki GSX 750 and a Kawasaki 600 - and a brown Mitsubishi Outlander.
Police said one of the motorcyclists, a 33-year-old man, was pronounced dead at the scene. The other motorcyclist was taken to hospital with a serious leg injury.
A spokesperson for Sussex Police said: “Emergency services were called to reports of a collision in Fox Way, Portslade, at 4.35pm on Saturday (April 16).
“Police closed the road in both directions while paramedics treated a motorcyclist, who was sadly pronounced dead at the scene.
“A second motorcyclist suffered a leg injury in the collision.
“Anyone who saw what happened, or who may have captured any relevant dash cam footage, is asked to contact police online or by calling 101 quoting Operation Euston.”
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Emergency services at the scene. Picture by Eddie Mitchell SUS-220417-090951001
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Emergency services at the scene of the collision. Picture by Eddie Mitchell SUS-220417-090901001
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Emergency services at the scene of the collision. Picture by Eddie Mitchell SUS-220417-090911001
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The air ambulance was sent to the scene of the collision. Picture by Eddie Mitchell SUS-220417-090921001