In the mathematical classification of finite groups, a thin group is a finite group such that for any odd prime the Sylow p-subgroups of the 2-local subgroups are cyclic.

defined thin groups and classified those of characteristic 2 type in which all 2-local subgroups are solvable.
The thin simple groups were classified by . The list of finite simple thin groups consists of:

• The projective special linear groups PSL₂(q) and PSL₃(p) for p = 1 + 2^a3^b and PSL₃(4)
• The projective special unitary groups PSU₃(p) for p =−1 + 2^a3^b and b = 0 or 1 and PSU₃(2ⁿ)
• The Suzuki groups Sz(2ⁿ)
• The Tits group
  Tits group
  In mathematics, the Tits group 2F4′ is a finite simple group of order 17971200 = 211 · 33 · 52 · 13 found by ....
  
   ²F₄(2)'
• The Steinberg group
  Steinberg group
  In mathematics, Steinberg group means either of two distinct, though related, constructions of the mathematician Robert Steinberg:*Steinberg group St in algebraic K-theory.*Steinberg group is a 'twisted' group of Lie type....
  
   ³D₄(2)
• The Mathieu group
  Mathieu group
  In the mathematical field of group theory, the Mathieu groups, named after the French mathematician Émile Léonard Mathieu, are five finite simple groups he discovered and reported in papers in 1861 and 1873; these were the first sporadic simple groups discovered...
  
   M₁₁
• The Janko group J1
  Janko group J1
  In mathematics, the smallest Janko group, J1, is a simple sporadic group of order 175560. It was originally described by Zvonimir Janko and was the first sporadic group to be found since the discovery of the Mathieu groups in the 19th century...