Does the property flipping rule apply if I sell a rental property I've only owned for six months?

Very likely, yes. Owning a residential property for only six months before selling it falls well within the short holding period the flipping rule targets, so unless one of the specific life-event exceptions applies to your situation, a sale after six months of ownership would generally be caught by the rule. That means any gain would be deemed fully taxable business income rather than a capital gain, and the principal residence exemption wouldn't be available even if you used the property as your home during that time.

This applies to rental properties as well as homes used personally, since the rule turns on how long you owned the residential property, not on what you were using it for. If none of the recognized life-event exceptions genuinely apply, things like a job relocation, a health issue, or the end of a relationship, covered in a related question, a six-month hold is squarely inside the zone this rule was designed to catch, and there's no separate exception simply for having a legitimate business or investment reason for selling. It's worth reviewing your specific circumstances against the exception list carefully before assuming normal capital gains treatment will apply.