Th is one of the 26 sporadic groups and was found by John G. Thompson (1976) and constructed by Smith (1976). They constructed it as the automorphism group of a certain lattice in the 248-dimensional Lie algebra of E₈. It does not preserve the Lie bracket of this lattice, but does preserve the Lie bracket mod 3, so is a subgroup of the Chevalley group E₈(3). The subgroup preserving the Lie bracket (over the integers) is a maximal subgroup of the Thompson group called the Dempwolff group (which unlike the Thompson group is a subgroup of the compact Lie group E₈).

The centralizer of an element of order 3 of type 3C in the Monster group is a product of the Thompson group and a group of order 3, as a result of which the Thompson group acts on a vertex operator algebra over the field with 3 elements. This vertex operator algebra contains the E₈ Lie algebra over F₃, giving the embedding of Th into E₈(3).

The full nomralizer of a 3C element in the Monster group is S₃ × Th, so Th centralizes 3 involutions alongside the 3-cycle. These involutions are centralized by the Baby monster group, which therefore contains Th as a subgroup.

The Schur multiplier and the outer automorphism group of the Thompson group are both trivial.

Conway and Norton suggested in their 1979 paper that monstrous moonshine is not limited to the monster, but that similar phenomena may be found for other groups. Larissa Queen and others subsequently found that one can construct the expansions of many Hauptmoduln from simple combinations of dimensions of sporadic groups. For Th, the relevant McKay-Thompson series is ${\displaystyle T_{3C}(\tau )}$  (OEIS: A007245),

${\displaystyle T_{3C}(\tau )={\Big (}j(3\tau ){\Big )}^{1/3}={\frac {1}{q}}\,+\,248q^{2}\,+\,4124q^{5}\,+\,34752q^{8}\,+\,213126q^{11}\,+\,1057504q^{14}+\cdots \,}$ 

and j(τ) is the j-invariant.

Linton (1989) found the 16 conjugacy classes of maximal subgroups of Th as follows:

• 2₊¹⁺⁸ · A₉
• 2⁵ · L₅(2)   This is the Dempwolff group
• (3 x G₂(3)) : 2
• (3³ × 3₊¹⁺²) · 3₊¹⁺² : 2S₄
• 3² · 3⁷ : 2S₄
• (3 × 3⁴ : 2 · A₆) : 2
• 5₊¹⁺² : 4S₄
• 5² : GL₂(5)
• 7² : (3 × 2S₄)
• 31 : 15
• ³D₄(2) : 3
• U₃(8) : 6
• L₂(19)
• L₃(3)
• M₁₀
• S₅

• Linton, Stephen A. (1989), "The maximal subgroups of the Thompson group", Journal of the London Mathematical Society, Second Series, 39 (1): 79–88, doi:10.1112/jlms/s2-39.1.79, ISSN 0024-6107, MR 0989921
• Smith, P. E. (1976), "A simple subgroup of M? and E₈(3)", The Bulletin of the London Mathematical Society, 8 (2): 161–165, doi:10.1112/blms/8.2.161, ISSN 0024-6093, MR 0409630
• Thompson, John G. (1976), "A conjugacy theorem for E₈", Journal of Algebra, 38 (2): 525–530, doi:10.1016/0021-8693(76)90235-0, ISSN 0021-8693, MR 0399193

• MathWorld: Thompson group
• Atlas of Finite Group Representations: Thompson group