It actually depends on the specifics of the job. If you work full-time but have a part-time side hustle, you may not need to list it at all. Assess whether the side hustle is relevant to your overall story and if it adds needed skills that will make you a better candidate for the job. If not, no need to list it. Your resume isn’t a biography, it’s more of a highlight reel. If the side hustle is relevant, you definitely should list it. If it’s clear from the nature of the job that it’s part-time -- like freelancing or script reading -- then you don’t need to qualify it. A hiring manager will see your primary job during the same period and make the assumption that the other position is part-time.
If you work two or more part-time jobs at the same time and plan to list both, it may be beneficial for you to include "(part-time)" next to your title. We see this a lot with personal assistants who manage two different clients, and it’s certainly worth clarifying to a future employer that you aren’t spreading yourself thin, but rather have been hired in a part-time capacity. However, just as is the case with a side hustle, if it's obvious that both of your jobs are part time -- like simultaneous internships -- a hiring manager will safely assume that each is part-time without you calling it out.
Lastly, if you are currently working only one part-time job, there’s no reason to draw attention to it on your resume. You may choose to acknowledge it in your cover letter as you explain why you’re looking to transition to full-time, add a second part-time position, or find a new part-time job, but there’s no rule that says you need to list part-time on yourresume. If you have the skills and are capable of doing the job you’re applying for, that’s all that matters.