The was a Japanese railway line in Hokkaido formerly operated by Hokkaido Railway Company (JR Hokkaido) which connects Fukagawa Station in Fukagawa and Mashike Station in Mashike. Following the discontinuation of service between Rumoi and Ishikari-Numata on 1 April 2023, the Rumoi Main Line was the shortest railway line in Japan to be classified as a "main line", at just .
On 19 November 2016, JR Hokkaido's president announced plans to further rationalize the network by up to , or approximately 50% of the network, including closure of the remaining section of the Rumoi Main Line (the Rumoi to Mashike section closed on 5 December 2016). This section of track from Ishikari-Numata to Rumoi was closed on 1 April 2023, with the final train running on 31 March 2023. The remaining track, between Fukagawa to Ishikari-Numata, closed on 1 April 2026, with the final train running on 31 March 2026.
The initial section of the line, between Fukagawa and Rumoi opened on 23 October 1910, and was extended to Mashike on 5 November 1921. At its peak, the line had a length of . The line was reclassified as a "main line" from 10 October 1931. With the privatization of Japanese National Railways (JNR) on 1 April 1987, the line came under the control of JR Hokkaido.
On 10 August 2015, JR Hokkaido informed the mayors of Rumoi and Mashike of its plans to close the section of the line from Rumoi to Mashike in 2016. In April 2016, it was officially announced that the section from Rumoi to Mashike would close, and the last service operated on 4 December of that year. The four towns and cities along the remaining line â Rumoi, Numata, Chippubetsu, and Fukagawa â began discussions with JR Hokkaido regarding the line's future, but Rumoi withdrew from the council due to having automobile infrastructure in place (due to the opening of the Rumoi Interchange on the Fukagawa-Rumoi Expressway in 2020, after which only 60 city residents used the train).
In September 2022, JR Hokkaido submitted a proposal to the Ministry of Land, Infrastructure, Transport and Tourism to close an additional between Ishikari-Numata and Rumoi. The closure was approved on 9 December 2022, end-of-service was announced for 31 March 2023, and the closure occurred on 1 April 2023.
Although the rest of the municipalities proposed that the rest of the line remains open, the proposal failed when JR Hokkaido demanded 300,000,000 yen yearly to keep the line open. As a result, the remaining section to Fukagawa closed on 1 April 2026. On the final timetable revision on 14 March 2026, through service via the Hakodate Main Line was abolished.
After closure, the Hidaka Main Line became the shortest "main line" in Japan, at 30.5 km.