In knot theory, a branch of mathematics, a twist knot is a knot obtained by repeatedly twisting a closed loop and then linking the ends together. (That is, a twist knot is any Whitehead double of an unknot.) The twist knots are an infinite family of knots, and are considered the simplest type of knots after the torus knots.

A twist knot is obtained by linking together the two ends of a twisted loop. Any number of half-twists may be introduced into the loop before linking, resulting in an infinite family of possibilities. The following figures show the first few twist knots:

• 
  One half-twist
  (trefoil knot, 3₁)
• 
  Two half-twists
  (figure-eight knot, 4₁)
• 
  Three half-twists
  (5₂ knot)
• 
  Four half-twists
  (stevedore knot, 6₁)
• 
  Five half-twists
  (7₂ knot)
• 
  Six half-twists
  (8₁ knot)

All twist knots have unknotting number one, since the knot can be untied by unlinking the two ends. Every twist knot is also a 2-bridge knot.^[1] Of the twist knots, only the unknot and the stevedore knot are slice knots.^[2] A twist knot with half-twists has crossing number . All twist knots are invertible, but the only amphichiral twist knots are the unknot and the figure-eight knot.

The invariants of a twist knot depend on the number of half-twists. The Alexander polynomial of a twist knot is given by the formula

and the Conway polynomial is

When is odd, the Jones polynomial is

and when is even, it is