The Thunder on Monday morning announced that Armstrong would not be taking up an option of another two seasons with the club in order to spend time with his young family.
In a statement, the club said it fully supported Armstrong’s decision.
“Sam has been a great leader of our team over the last three years, setting very high standards of professionalism in what has become a much more sophisticated and challenging league since the transition from SEABL to NBL1,” the club said.
“He has been a devoted servant of the club and a great ambassador for our organisation.
“He is highly thought of both internally and throughout the wider basketball community and has left a lasting impression on everyone, be they players, officials, volunteers, fans and even personnel from opposition teams.
“Sam is a life member of the club and one of our most celebrated team members over an outstanding playing career.
“He certainly brought that level of excellence across into the coaching sphere and he will certainly be missed as our off-court leader in the coming seasons.
“We are confident, however, that he will remain a staunch fan and will always offer support and an attentive ear for anyone requiring advice or feedback going forward.
“We wish him well in all his endeavours in the future and thank him wholeheartedly for his tremendous contribution across all aspects of our organisation over the past three seasons.”
The Thunder will “shortly initiate the process of advertising for a replacement coach”.