Emergency services were called to the scene, on the B9071 between the villages of Aith and Voe, at about 17:00 on Monday.
The man was taken to Gilbert Bain Hospital in Lerwick for treatment but Police Scotland said he died on Tuesday morning from his injuries.
The road was closed to allow a collision investigation to take place.
Sgt Donald MacKinnon said: "Our thoughts are with this man's family at this very sad time.
"I would appeal to anyone who was in the area around the time in question and saw either vehicle to please get in touch on 101 as we establish the circumstances surrounding this incident.
"The road will remain closed while a collision investigation team carry out enquiries at the scene and I would like to thank the public in advance for their patience."
A 60-year-old motorcyclist has died after a crash involving a truck in Shetland.