When can a taxpayer file married filing joint on a tax return?
The IRS considers allows you to file a joint tax return if you are legally married. Further, per the IRS definition, "you are considered married if you are legally married on the last day of the year."
The only requirement to file jointly is for both you and your spouse to agree to file a joint tax return, and both must sign the return. So, in your particular case, you are eligible to file a joint tax return.
Despite the choice available for married taxpayers to file a separate tax return, the IRS has specifically advised that, "to file a joint return, the tax liability may be lower than your combined tax for the other filing statuses."
So, I would recommend that since you qualify to file married filing jointly, you should consider to do so, as it may save you considerably in Federal Income tax liability.