Hilinski suffered an apparent self-inflicted gunshot wound to the head, and a rifle and suicide note were found next to him, according to Pullman police.
Officers responded to Hilinski's apartment to conduct a welfare check after he "did not show up for practice earlier in the day," police said.
"We are deeply saddened to hear the news of Tyler's passing," Washington State coach Mike Leach said in a statement. "He was an incredible young man and everyone who had the privilege of knowing him was better for it. The entire WSU community mourns as thoughts and prayers go out to his family."
Hilinski, a Claremont, California, native, just finished his redshirt sophomore season for the Cougars. He started the team's bowl game against Michigan State and was expected to take over the starting role next season. He enrolled at Washington State in January 2015 after attending Upland High School in Southern California.
"The tragic news today surrounding Tyler Hilinski is devastating to all. Tyler was a tremendous individual, great friend and teammate, and he will be deeply missed. Our hearts go out to his family and friends,'' interim athletic director John Johnson said Tuesday night in a statement.
"Earlier tonight, the football team was brought together and informed of the tragedy. There, they were met by campus and department counseling and psychological services, including athletics' on-staff clinical psychologist and a licensed mental health counselor, along with WSU Athletics medical team. The university will continue to coordinate and provide ongoing counseling care for all student-athletes as long as needed.''
Hilinski's younger brother, Ryan, is a highly regarded quarterback prospect in the Class of 2019.
Ryan Hilinski issued a statement later Tuesday.
"We, as a family, are grieving after receiving news that my brother, Tyler Hilinski took his own life this afternoon," the statement read. "We are in complete shock and disarray. Tyler was the kid that put a smile on everybody's face when they were down, especially his family. We will mourn the loss of Tyler for some time but celebrate the way he lived his life every day."
Ty you were a great teammate, friend, brother anything we needed you to be. You brought smiles to the people around you. I am sorry I could not be there for you when you needed a smile for yourself. Love you man. Rest In Peace🙏🏽 pic.twitter.com/LmNC1ZLNZi— Skyler Thomas (@Sky_Dolla_Sign) January 17, 2018
God, let Tyler find peace. Everyone please pray for the Hilinski family tonight. Heaven received a very special person. pic.twitter.com/mx4i1MoOhK— John Bledsoe (@johnbledsoe11) January 17, 2018
Former Washington State linebackers coach Roy Manning, who recently left for a position at UCLA, tweeted:
Words can't describe what I'm feeling right now. My heart is beyond saddened. Please pray for the family and all of us affected!— Roy Manning (@CoachRoyM) January 17, 2018
Hilinski appeared in eight games during his sophomore season, throwing for 1,176 yards and seven touchdowns. His most memorable outing came in the second week of the season when he led Washington State from a 21-point deficit in the fourth quarter to beat Boise State 47-44 in triple overtime. Hilinski came off the bench to throw for 240 yards and three touchdowns, and was carried off the field after the victory.
The Associated Press contributed to this report.