When people hear "massage," most of them think of wellness or of sore back muscles after sports. Buddhist massage comes from a different context. It's a form of bodywork that's understood as spiritual practice, and its roots go back, according to tradition, to the time of the historical Buddha.

The roots: Jīvaka 耆婆
According to Buddhist tradition, the Buddha's personal physician was a man named Jīvaka Komārabhacca, in Japanese Giba 耆婆. The Pāli texts describe him as a remarkably skilled doctor who also treated the monastic community, the sangha. To this day, several Asian traditions of bodywork, Thai massage for example, regard him as their founding father and honor him before treatments.
How exactly the bodywork of that time looked, we can't reconstruct in detail. What's clear is that care for the body had a place in monastic life, because long hours of sitting meditation are hard on the body. As Buddhism spread across Asia, from the 3rd century BCE onward under King Aśoka and later along the trade and pilgrimage routes, medical knowledge traveled along with the monks. In each country it mixed with local traditions, with Chinese medicine, with Daoist ideas of energy and with the ritual practices of esoteric Buddhism. In other words, there isn't one single Buddhist massage, but a whole family of related traditions.
Touch as meditation 觸行
What makes Buddhist massage different from a sports massage isn't so much the movements as the intention and the inner attitude of the person giving it. You touch with the same attention you'd bring to a mantra or to calligraphy, and so the touch itself becomes meditation, for both sides.
As I practice it, there are two levels. The basic Buddhist massage works with mindful touch, stretching and pressure, with or without oil, and also with direct body contact. The advanced tantric-Buddhist massage adds mantras of esoteric Buddhism and ritual elements, and it's opened through initiation.

Aizen Myōō, the Wisdom King of Love 愛染明王
In this context you'll meet a figure who may surprise you at first, Aizen Myōō 愛染明王. His name means something like "the Wisdom King dyed in love" or "stained by passion." He's depicted in red, with six arms, a lion's head in his crown of flames, and bow and arrow in his hands. He doesn't look like someone who has anything to do with massage, which is exactly what makes him interesting.
The Wisdom Kings (myōō 明王) transform what would otherwise be an obstacle into spiritual power. Aizen Myōō does this with passion and desire, which he doesn't suppress but transforms into the longing for awakening. Shingon expresses this with the phrase bonnō soku bodai 煩悩即菩提, "the passions are themselves awakening." Put simply, the body isn't seen as the enemy of the mind, but as its ally.
In Shingon Reiki, Aizen Myōō is the protector of Buddhist massage, and the initiation into Aizen Myōō is part of the Master Path. Daoist influences also play a role, especially the work with the body's energy channels, which has its own long tradition in China and Japan.
Why touch can be spiritual 身心
Kūkai, the founder of Shingon, summed up his teaching in the term sokushin jōbutsu 即身成仏, "becoming a Buddha in this very body." This means awakening in this life and not after death, and not by rejecting the body but with it. For Kūkai, the body is part of the path, which is why mudras, the gestures of the hands, play such a large role in Shingon.
Buddhist massage follows the same idea. When touch opens the body, the mind often opens with it, and when the muscles let go, mental tension frequently eases as well. This is how many people experience it, and it fits with the practice of spinal Reiki, which also works with the life force along the spine.

In Shingon Reiki, Buddhist massage isn't passed on as an isolated technique. It's part of the Master Path and connects body, mind and ritual. Have you ever experienced touch that felt more like meditation than like treatment?
Discover the Master Path
Buddhist massage is part of the Master Path in Shingon Reiki. If you like, have a look at which entry fits you.
Your Path in Shingon Reiki Rituals in Reiki