Assistant coach Paul Holohan will take over the program as the interim head coach for the remainder of the 2007 campaign.
"It's been a wonderful ride here at UC San Diego," said Armstrong. "There was no program here when I started and to see all of the changes that have taken place during the last 26 years has been amazing."
Armstrong's announcement comes just eight matches into his 26th season as head coach at UC San Diego. The 2007 squad has posted a 1-6-1 overall record with four of the six losses coming by one goal. The '07 Tritons are starting a very young nucleus, consisting of five freshman, three sophomores and three juniors.
"I only wish my health was good enough for me to finish this season because I really enjoy working with this group of young men," added Armstrong. "They have the potential to turn into a very formidable team in the not too distant future."
Armstrong, who originally hails from England, guided the Triton men's program to three NCAA Division III championships (1988, 1991 and 1993) and a 322-135-46 overall record. UCSD is now a D2 program.
"We all were caught off guard and saddened by Derek's announcement today," said Director of Athletics Earl W. Edwards. "As much as we will miss his demeanor and class on the sidelines, we want nothing more than for him get healthy and enjoy spending time with his wife, children and grandchildren. His contributions as a head coach and an ambassador for the university and collegiate soccer will be greatly missed."
Armstrong's influence has extended beyond the UCSD campus. He has been director of coaching for the Nomads Soccer Club, one the first clubs to aggressively develop a youth program.
When U.S. Soccer began to ramp up its youth program 20 years ago, Armstrong was picked to coach the U-20 national team that qualified for the 1987 World Youth Championship in Chile.
He served as head coach of the senior Nomads when they won the WSL title in 1987 and 1989.
And he is a founding member of U.S. Club Soccer.