Am I legally obligated to file my ex's tax return?

You are not obligated to file her tax return and unless there is a contrary provision in a court order or agreement, your ex has no obligation to provide you with a copy of your ex's tax return.

If you become divorced during the calendar year, then you should file as single for that calendar year. There is no right for you to inspect your ex-spouse's return, or vice versa. As individuals who are filing separately and have no marital relationship with one another, you can not legally force the other person to submit their return to you for review. If you feel that your spouse may attempt something that would affect your return, such as claiming dependents that you should be able to claim, then you should file your return as soon as possible. Additionally, you will want to mail a paper return instead of electronically filing, as the I.R.S. will usually allow paper returns to take precedence over electronically filed returns. Essentially, if you file before your ex-spouse, and if you file a paper return, then the I.R.S. should accept your return and audit your ex-spouse's if there is any inconsistent or contradictory information. Even if your ex-spouse files his or her return first, and the I.R.S. audits your return as a result, you should still be able to submit enough information or documentation showing that your return is accurate, and thus that your return should be accepted and your ex-spouse's should be audited instead.