Rubio is a Democrat whose district stretched from Fresno to Bakersfield.
Rubio cited a desire to spend more time with his family as the primary reason. But he's also leaving to take a high profile job with Chevron oil.
And his departure comes at the same time fellow democrats have questioned his efforts to weaken state environmental laws.
Michael Rubio was elected to the state senate in 2010. Seen as a rising star in the party. Fresno County Democratic Party Chairman Michael Evans says his departure was not expected.
"It was definitely a surprise; we thought he might even be looking at the congressional seat we certainly didn't think he was looking at resignation."
Rubio's district covered about half of the people in the city of Fresno and a quarter of Bakersfield's population. Along with parts of Tulare County and all of Kings County.
In the legislature he was known to work with the other party. Republican assembly member Jim Patterson of Fresno says Rubio was a key force in getting water to the valley, and working to ease environmental regulations.
"I think this is a big loss for leadership for the valley that moderate democrats and we republicans could support.
Rubio's efforts to relax the California environmental quality laws set him at odds with the Senate leadership. But in a statement Rubio said he's leaving because,
"I have realized that my current professional path has left little opportunity to be home for those who are most important to me, which is why I am making a change,''
Sacramento Political Analyst Steve Mavigilio says Rubio's decision to go to work for an oil company raises questions.
"Some people will make the link that he was behind a lot of changes in environmental laws because he felt like he was going to go to work for chevron. I don't know if that's true, it would be highly doubtful.
Rubio's departure will leave Democrats one vote short of the supermajority they had in the Senate, but that's not expected to last. Two other state senate seats where democrats are favored will be filled by special elections next month. Democrats lead Republicans in the 16th district by a 20 point margin, but Evans says the party isn't taking anything for granted.
"It definitely won't be a cakewalk; in a special election Republicans typically have higher turnouts than Democrats. But with a good candidate and a good ground game it's definitely winnable." Assembly Member Henry T. Perea, a Democrat from Fresno is already being mentioned as a likely candidate.