Moria gives it a full 5 stars, calls it "one of the most extraordinary pieces of underground filmmaking to have emerged in the 1980s," and says,
The underlying theme of much of Shinya Tsukamoto’s work is similar to that in the films of David Cronenberg – flesh has become a mutable battleground where the age-old Manichean debate has finally found its warring ground. In Tetsuo – The Iron Man, flesh represents the downbeaten self, which is engaged in a war with machinery. Tsukamoto sexually fetishizes machinery – pistons, oil, the gleam of chromium, jagged edges, tangled wires – and equates it with wildly repressed desires – it is constantly trying to burst from inside human skin, run rampant and absorb everything into its mass. What more potent an image can such a regimented society as Japan have produced than that of a white-collar worker engaged in a battle of wills to stop his flesh being taken over by machinery that insists on erupting from within?1000 Misspent Hours calls it "a mindfuck par excellence". 366 Weird Movies begins their review with this: "Attempts to describe Tetsuo: The Iron Man to the uninitiated run up against a problem of missing touchstones—what other irrational gore movies about men transforming into machines from the inside out can you compare it to?" HorrorNews.net has screen shots and calls it "a brilliant piece of filmmaking that combines nightmare imagery with modern industrial sensibilities."
Rotten Tomatoes has a 77% critics score.