UFC fighter Rachael Ostovich's MMA fighter husband has been arrested for ATTEMPTED MURDER days after allegedly breaking his wife's face ... and he's still behind bars in a Honolulu jail.

TMZ Sports broke the story ... Ostovich -- who had to pull out of her scheduled January fight with Paige VanZant -- was hospitalized this weekend with serious injuries -- including a broken orbital bone -- after her husband allegedly assaulted her during an argument at their home.

Cops in Honolulu opened a domestic violence investigation earlier this week ... and now police officials tell us 27-year-old Arnold Berdon -- Ostovich's husband and father of her child -- was arrested Tuesday evening on suspicion of 2nd degree attempted murder.

Berdon -- a fighter himself who holds a 7-2 record -- is still in custody at the Honolulu PD main station.

He has not yet appeared before a judge ... where bail could be set.

If he is released ... Rachael's husband won't be allowed to contact Ostovich -- 'cause she got a restraining order ... barring contact.

For his part ... a family member of Arnold told us he intends to vigorously dispute in court any allegations he assaulted his wife.