The Tōkaidō Main Line (東海道本線, Tōkaidō-honsen^?) is the busiest trunk line of the Japan Railways Group (JR Group), connecting Tōkyō and Kōbe stations. It is 589.5 km (366.3 mi) long, not counting its many freight feeder lines around the major cities. The high-speed Tōkaidō Shinkansen runs largely parallel to the line.

The term "Tōkaidō Main Line" is largely a holdover from pre-Shinkansen days; now various portions of the line have different names which are officially used by JR East, JR Central, and JR West. Today, there are no passenger trains that operate over the entire length of the line (other than certain overnight services; see below), so longer intercity trips require several transfers along the way.

The Tōkaidō Main Line is owned and operated by three JR companies:

The Tōkaidō route takes its name from the ancient road connecting the Kansai region (Kyoto, Osaka) with the Kantō region (Tokyo, then Edo) through the Tōkai region (including Nagoya.) Literally, it was the Tōkai road, or Road through Tōkai. The Tōkaidō line does not follow the old road exactly, since the latter diverges at Nagoya toward the Mie Prefecture coastline; to follow it by train, the Kansai Main Line and Kusatsu Line would have to be followed from Nagoya to Kusatsu. The largest population centers in Japan are along this route - Tokyo, Yokohama, Nagoya, Kyoto, Osaka and Kobe. These centers have grown to occupy an ever more dominant role in the country's government, financial, manufacturing and cultural life.

Mainline, Main line, or Main Line may refer to: