The crash occurred between Lucky Peak Dam and the Diversion Dam.
The Ada County Sheriff's Office says the victim is a 51-year-old Boise man.
Deputies say he was riding south on the highway when he went off the right shoulder, hit a basketball-sized boulder, and crashed.
The man wasn't wearing a helmet, and investigators say he suffered extensive head injuries and died instantly.
The highway was reduced to one lane in the area while the crash was being investigated.
More information will be reported here once it's available.