Can I make a priority claim when filing a trademark application?

Only a priority claim based on a treaty may be made.
The treaty-based priority system under the Trademark Act (Article 20 of the Trademark Act) is a system under which, where a national of a State party that grants a priority right with respect to trademark registration applications in Korea by treaty files a trademark registration application (hereinafter the "prior application") in that State party or another country (the first country) and then files an application for registration of the same trademark in Korea (the second country) (hereinafter the "subsequent application") and claims priority, the point in time for applying certain provisions of the Trademark Act (Article 8, first-to-file principle) to the subsequent application is deemed to be the filing date of the prior application; this principle also applies in the same way where a Korean national files a prior application in one of the above treaty States party and files a subsequent application for the same trademark in Korea and claims priority (Article 20, Paragraph 1 of the Trademark Act).

A person who wishes to claim priority cannot do so unless the application is filed within 6 months from the filing date of the first application that forms the basis of the priority claim, and a person who wishes to claim priority must, when filing the trademark registration application, state in the trademark registration application form the purport thereof as well as the name of the country in which the first application was filed and the date of that application.

A person who has claimed priority must submit to the Commissioner of KIPO (Korean Intellectual Property Office), within 3 months from the filing date of the trademark registration application, a document stating the date of the trademark registration application as recognized by the government of the country in which the first application was filed, together with a certified copy of the trademark and the designated goods; if the above documents are not submitted within the period, the priority claim loses its effect.