After filing my 2024 tax return, I received a 1099-K for rental income from a timeshare. What expenses am I allowed to deduct against the 1099 income?

If the timeshare was rented for 14 days or less, you don't need to do anything.  It is not income and you've already filed your return.

If the timeshare was rented for more than 14 days, you need to report the rental income on Schedule E.  

You'll need to allocate the costs based on the days it was available for rent.  Even though you did not use it at all during 2024, if you did not make available certain times, those are considered personal use days.  Then, you can prorate the costs between personal and business use.  Most ordinary and necessary expenses you have incurred is allowed to be expensed- maintenance fees, cleaning, management fees, etc.

Here is some information you may find helpful: 10 Tax Tips for Airbnb, HomeAway & VRBO Vacation Rentals

MindyB: Thanks for your response, but why would I have to allocate expenses; based on what? I am given enough "star options" (points) each year by Vistana/Marriott to reserve two to three weeks depending on the location and time of year. I'm entitled to reserve eight different "home" resorts and many others with differing reservation periods. I pay a maintenance fee (approx. $2500) for the use of those weeks. If I rent those weeks, and it's not over 14 days, you're saying I don't have to report it at all? Is that the case even though I did not use my timeshare for personal use in 2024? Then you talked about allocating costs, so that's where you lost me.  I have nothing to do with making them "available" other than my two weeks. So the maintenance fees cover those two weeks. Why would I have to consider any kind of allocation?