"The disciplinary board has decided to hand down a warning to (Björn) Hurtig. It was not a united decision," Bar Association Secretary General Anne Ramberg told AFP.
The warning, she said, was "in response to the criticism directed to him by an English judge".
Ramberg said some of the disciplinary board's members had wanted to issue Hurtig a warning and a fine, but he was given only a warning. She refused to give more details but said such a warning had no impact on Hurtig's practising.
In a February 25 letter, the association had asked Assange's lawyer to explain ethics breach allegations made by the British judge in charge of the WikiLeaks founder's extradition hearing.
When the London court had ruled that Assange could be extradited to Sweden for questioning on allegations of rape and sexual molestation, judge Howard Riddle had among other things accused Hurtig of making a "deliberate attempt to mislead the court".
Riddle was referring to testimony in which Hurtig had said he had been unable to contact Assange last year when he was sought by Swedish prosecutors for questioning.
"The judge has made rather serious accusations in regard of Mister Hurtig and we wanted him to respond to that," Ramberg told AFP at the time the Bar Association sent him the letter.
Reached by AFP Thursday, Hurtig said he did not want to comment the case.
Assange, whose WikiLeaks website gained notoriety for releasing thousands of secret US cables, is in Britain awaiting the appeal he lodged following the British court's decision that he could be sent to Sweden.