My wife and I have been separated for a few years but we are still legally married in the state of Utah. Is it legal for her boyfriend to claim our son on his taxes? I don't pay child support and our son lives with my wife and her boyfriend during the week and is with me every weekend. She isn't filing any taxes, so I wanted to claim my son on my taxes, but she is arguing and saying that I can't because her boyfriend is claiming him. Is this legal?

“ Is it legal for her boyfriend to claim our son on his taxes?”---->I guess this is not an issue of legality. Your son is a dependent of a custodial parent, not your wife’s boyfriend’s .However, he can claim his g/f and the kid dependents (as Qualifying Relative) only if g/f and the kid lived with him all the year, g/f AND THE KID did not earn more than $3,650, he has provided more than half of her total support for her and the kid, and the relationship does not violate local law. So, he can file as single and claim $3.650 exemption from each person as his dependents. However, he CANNOT file HOH, claim child care, child tax or EIC
“ I don't pay child support and our son lives with my wife and her boyfriend during the week and is with me every weekend.”----> Then your kid is NOT your dependent.
“ She isn't filing any taxes”----> Then she can’t claim the kid as her dependent UNLESS she files her own return. But as said above, as long as her boyfriend satisfies the requirements, then he can claim both his g/f and the kid as his dependents.
“, so I wanted to claim my son on my taxes, but she is arguing and saying that I can't because her boyfriend is claiming him. Is this legal?”----> This is not an issue of legality; both of you CAN”T claim the kid as your dependent. You can’t claim the kid as your dependent because as said above you are not a custodial parent of the kid. And you do not provide more than half of the kid's living expenses during the year. And the kid didn’t live you more than six months during the year. A custodial parent is the person who has had primary custody of a child for most part the year. However, you may still be able to claim the child as your dependent if the custodial parent signs a Form 8332, Release of Claim to Exemption, or a form containing all of the same information listed on this form and you can satisfy the requirements; living together with the kid or providing more than half of the support etc.. By completing Form 8332, the custodial parent would be indicating that she wishes to release her claim to a dependency exemption for the child on the form. Without this form, you generally cannot claim the child who did not live with you as a dependent because the child is a qualifying child of someone else, his mom in this particular case.