The tribunal said Ryder, who has been in self-imposed exile from international cricket since February 2012, tested positive after a domestic match in March this year. (Agencies)
It said Ryder's sample showed traces of the stimulants PBA and DEBEA and the batsman admitted he took two supplement tablets in the days before the test.
The disciplinary body said it accepted Ryder's argument that he was not aware the over-the-counter product contained banned substances and he took it because he was struggling with his weight, not to enhance his on-field performance.
However, it said he did not check with authorities whether the supplement was legal, even though the label warned it might contain substances outlawed by some organisations.
Instead, Ryder and his trainer did their own research on the Internet and concluded it was safe.
The tribunal said that, as a professional athlete, Ryder was obliged to be more cautious before taking a potentially illegal supplement.
"The tribunal thought a penalty of six months suspension was also appropriate," it said, pointing out the maximum punishment available was two years.
The ban was backdated to April, meaning Ryder will be available to play again in October.
The 29-year-old is one of New Zealand's best batting talents, averaging 40.93 in 18 Tests, but his career has been marred by disciplinary lapses and off-field problems.
He is still recovering from an assault outside a bar in the South Island city of Christchurch in March, which derailed his plans to return to the Indian Premier League.
Ryder walked away from international cricket last year, saying he needed to sort out "personal issues" and has so far resisted calls from pundits and fans for his return. New Zealand Cricket declined to comment.
The tribunal said Ryder, who has been in self-imposed exile from international cricket since February 2012, tested positive after a domestic match in March this year.