Officers and medics responded to the scene, at Second Avenue and University Street, at about 9 a.m. after receiving reports of a serious accident.
The bicyclist was found dead at the scene, said Jonah Spangenthal-Lee of the Seattle police.
Witnesses said the bicyclist was heading south on Second Avenue in the left-hand bike lane at the same time as the truck was heading south in the left traffic lane. The truck then turned left on University Street and struck the bicyclist, witnesses said.
The truck driver, a man in his 40s, stopped after the collision and was interviewed by p0lice. There was no indication that he was impaired at the time of the crash, police said.
"They just did not see each other, and it happened so quickly where the biker just got run over by the truck and was completely unresponsive immediately afterwards," said witness Daniel George. "It was unbelievable. It happened so quickly. It was a tragedy."
George said it appeared the truck driver didn't realize what had happened at first.
"He got out of the truck immediately, and he was just completely distraught when it happened," George said. "All you heard was a bang and the next thing you know, the biker was face down on the street.
"It was just bad. It was terrible. I don't know how to react. I still don't know how to react," he said.
Detective Patrick Michaud of the Seattle police said investigators want to know whether the bicyclist was in the truck driver's blind spot when he turned.
"That's very possible, and we're going to look into that," Michaud said. "When we do these kinds of investigations, we take the vehicle that was involved, and the bike, and we do measurements, and we can figure out exactly where people were, and we can determine if the person was, in fact, in the blind spot of the vehicle."
He added that the bicycle had the right-of-way in this case.
"When you're making a left turn, you have the duty to make sure you are clear to do so," Michaud said. "The bikes have the right-of-way to be able to go straight, even if you are making a left turn and you are on the left side, so you have to clear the turn before you can make it."
Antoine McNamara, a bicyclist who sometimes rides along Second Avenue, said it is a very dangerous place for cyclists.
"I know they're supposed to open a new bike lane in about a week or two, and it's just such sad timing that we couldn't get on this earlier," McNamara said. "This is an incredibly dangerous bike lane ... you bike right through the door zone. ... I never actually use the lane because it's so unsafe. I take the actual traffic lane."
"I've always been told to assume that the drivers can't see you - that they're texting and that they're drunk, because at least one of those is probably true," he added.