By Chen Chao-fu, Yang Ssu-jui,
Kuo Chu-chen and Frances Huang
The by-election was held after a recall vote in early June to remove the KMT's Han Kuo-yu (韓國瑜), who won election in November 2018 and ended the DPP's 20-year hold on the city.
Chen lost the 2018 race by a 9 percent margin, but after Saturday's victory, he will serve out the remainder of the term, which ends in December 2022.
Before the by-election, President Tsai Ing-wen (蔡英文), who doubles as the DPP Chairwoman, urged the party's members to unite behind Chen to bring back "the glory of Kaohsiung."