I'm an Indonesian citizen holding an US visa. My itinerary is USA -> China -> South Korea -> China -> USA. All within 2 weeks. Do I need to apply for a South Korean visa?
In general, you need a visa as an Indonesian passport holder. The only exemption, which applies to you, is where you have a re-entry visa and a travel plan including travel tickets to a country like the USA or Canada.
If you have a re-entry visa to the US, and you do not plan to stay in Korea longer than 30 days, you can enter visa-free as a Transit tourist, bound for the USA as a holder of a re-entry visa to the USA.