Update:
As of this week (written on September 1st, 2013), Domestic Abuse has officially been outlawed in Saudi Arabia.
In a historic move, Saudi Arabia banned domestic abuse this week, outlawing any form of abuse at home or in the workplace.
The Council of Ministers' approval of the law Monday marks the first time the country has officially recognized domestic violence as a punishable crime. Offenders may face up to a year in prison and a maximum fine of 50,000 Saudi riyal (about $13,300), the Saudi Gazette reports.
The law will also provide shelter for victims of abuse and ensure the identities of those who report it are kept confidential.
The legislation makes sexual violence in the home and the workplace a punishable crime. Moreover, it provides for shelter for victims of abuse and places the onus on law enforcement agencies to follow up on reports of abuse. It is backed up by a jail sentence and hefty fines.
Saudi Arabia's cabinet has passed a ban on domestic violence and other forms of abuse against women for the first time in the Kingdom's history.
The cabinet approved the ban on physical or sexual violence earlier this week, which applies both at home or within the work place.
The legislation makes domestic violence a punishable crime for the first time. It also provides treatment and shelter for victims of abuse and holds law enforcement agencies accountable for investigating and prosecuting allegations of abuse.
The ban includes penalties of a maximum12 month jail sentence and fines of up to $13,000.
“All civilian or military employees and all workers in the private sector who learn of a case of abuse — by virtue of their work — shall report the case to their employers when they know it,” Alnbawaba reported the cabinet as saying in a statement. “The employers shall report the case to the Ministry of Social Affairs or police when they know it.”
also: Saudi Gazette
In a video done on this in the Young Turks it's also said that there is an anonymous hot-line, I wasn't able to find it mentioned in other sources.
Partially Outdated Information:
It's legal in Saudi Arabia, but the situation seems to be improving.
According to a source at MSN News which depicts an anti domestic violence campaign:
There are currently no laws in Saudi Arabia that protect domestic violence victims.
Also from this source:
There is not one law in Saudi Arabia that regards violence toward women as an illegal activity
...
In terms of law, there is not one law in Saudi Arabia that regards violence toward women as an illegal activity. As a matter of fact, women in Saudi Arabia are minors under law until their death, making it impossible for a woman to make any decision on her own without the permission of her guardian.
This means that even if a woman is “radical” enough to disobey her guardian or reject his abuse or decisions, she has got nowhere to go. She cannot file a complaint or leave the country or do practically anything without her guardian’s permission, which in most cases, ironically, is the abuser himself.
...
“In May, Jeddah’s Summary Court convicted a man for physically abusing his wife to the point of hospitalization, but sentenced him to learning by heart five parts of the Quran and 100 sayings of the Prophet Muhammad.”
The legal system in Saudi Arabia does not have the sufficient tools and laws that would deter men from domestic violence. In case of drugs and murder, the punishment is death penalty, which makes people think twice before committing such crimes because of the consequences. Yet, when it comes to domestic violence the punishment is learning parts of the Quran and sayings of the Prophet Muhammad.
and just recently a national campaign was started by an NGO of the Royal Family to stop domestic violence.
However, in addition to the cases depicted in @non-Saudi man and @user1873 answers, the King Khalid Foundation started a national campaign in Saudi Arabia aimed at stopping domestic abuse.
Here are sources describing the campaign:
The site of the KKF states:
KKF did a research on “Women and Child Abuse Prevention Law” which has been adopted by the Government of Saudi
Also, a Foreign Policy article on the campaign mentions another recent campaign, the "White Ribbon Campaign" which is part of the global White Ribbon Campaign against domestic violence:
The launch of this initiative is all the more striking considering the start of another campaign that preceded it by just a few days. This is the "White Ribbon campaign," the Saudi version of the international campaign of the same name, which aims to involve men more actively in efforts to stop violence against women.
Considering the events of husbands being criminally prosecuted and punished for domestic abuse do start to happen, and a national campaign by a Royal Family NGO, that claims to affect the government, It can be said that Domestic Violence in Saudi Arabia will, hopefully, be on the decline, and its prosecution on the rise.