diff --git a/Mathlib.lean b/Mathlib.lean
index 11e1a787c07d8..7717f18232c7e 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -539,6 +539,7 @@ import Mathlib.Combinatorics.SetFamily.Kleitman
 import Mathlib.Combinatorics.SetFamily.LYM
 import Mathlib.Combinatorics.SetFamily.Shadow
 import Mathlib.Combinatorics.SimpleGraph.Acyclic
+import Mathlib.Combinatorics.SimpleGraph.AdjMatrix
 import Mathlib.Combinatorics.SimpleGraph.Basic
 import Mathlib.Combinatorics.SimpleGraph.Clique
 import Mathlib.Combinatorics.SimpleGraph.Coloring
diff --git a/Mathlib/Combinatorics/SimpleGraph/AdjMatrix.lean b/Mathlib/Combinatorics/SimpleGraph/AdjMatrix.lean
new file mode 100644
index 0000000000000..5b68eb9b03725
--- /dev/null
+++ b/Mathlib/Combinatorics/SimpleGraph/AdjMatrix.lean
@@ -0,0 +1,300 @@
+/-
+Copyright (c) 2020 Aaron Anderson. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Aaron Anderson, Jalex Stark, Kyle Miller, Lu-Ming Zhang
+
+! This file was ported from Lean 3 source module combinatorics.simple_graph.adj_matrix
+! leanprover-community/mathlib commit 3e068ece210655b7b9a9477c3aff38a492400aa1
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.Combinatorics.SimpleGraph.Basic
+import Mathlib.Combinatorics.SimpleGraph.Connectivity
+import Mathlib.LinearAlgebra.Matrix.Trace
+import Mathlib.LinearAlgebra.Matrix.Symmetric
+
+/-!
+# Adjacency Matrices
+
+This module defines the adjacency matrix of a graph, and provides theorems connecting graph
+properties to computational properties of the matrix.
+
+## Main definitions
+
+* `Matrix.IsAdjMatrix`: `A : Matrix V V α` is qualified as an "adjacency matrix" if
+  (1) every entry of `A` is `0` or `1`,
+  (2) `A` is symmetric,
+  (3) every diagonal entry of `A` is `0`.
+
+* `Matrix.IsAdjMatrix.to_graph`: for `A : Matrix V V α` and `h : A.IsAdjMatrix`,
+  `h.to_graph` is the simple graph induced by `A`.
+
+* `Matrix.compl`: for `A : Matrix V V α`, `A.compl` is supposed to be
+  the adjacency matrix of the complement graph of the graph induced by `A`.
+
+* `SimpleGraph.adjMatrix`: the adjacency matrix of a `SimpleGraph`.
+
+* `SimpleGraph.adjMatrix_pow_apply_eq_card_walk`: each entry of the `n`th power of
+  a graph's adjacency matrix counts the number of length-`n` walks between the corresponding
+  pair of vertices.
+
+-/
+
+
+open BigOperators Matrix
+
+open Finset Matrix SimpleGraph
+
+variable {V α β : Type _}
+
+namespace Matrix
+
+/-- `A : Matrix V V α` is qualified as an "adjacency matrix" if
+    (1) every entry of `A` is `0` or `1`,
+    (2) `A` is symmetric,
+    (3) every diagonal entry of `A` is `0`. -/
+structure IsAdjMatrix [Zero α] [One α] (A : Matrix V V α) : Prop where
+  zero_or_one : ∀ i j, A i j = 0 ∨ A i j = 1 := by aesop
+  symm : A.IsSymm := by aesop
+  apply_diag : ∀ i, A i i = 0 := by aesop
+#align matrix.is_adj_matrix Matrix.IsAdjMatrix
+
+namespace IsAdjMatrix
+
+variable {A : Matrix V V α}
+
+@[simp]
+theorem apply_diag_ne [MulZeroOneClass α] [Nontrivial α] (h : IsAdjMatrix A) (i : V) : ¬A i i = 1 :=
+  by simp [h.apply_diag i]
+#align matrix.is_adj_matrix.apply_diag_ne Matrix.IsAdjMatrix.apply_diag_ne
+
+@[simp]
+theorem apply_ne_one_iff [MulZeroOneClass α] [Nontrivial α] (h : IsAdjMatrix A) (i j : V) :
+    ¬A i j = 1 ↔ A i j = 0 := by obtain h | h := h.zero_or_one i j <;> simp [h]
+#align matrix.is_adj_matrix.apply_ne_one_iff Matrix.IsAdjMatrix.apply_ne_one_iff
+
+@[simp]
+theorem apply_ne_zero_iff [MulZeroOneClass α] [Nontrivial α] (h : IsAdjMatrix A) (i j : V) :
+    ¬A i j = 0 ↔ A i j = 1 := by rw [← apply_ne_one_iff h, Classical.not_not]
+#align matrix.is_adj_matrix.apply_ne_zero_iff Matrix.IsAdjMatrix.apply_ne_zero_iff
+
+/-- For `A : Matrix V V α` and `h : IsAdjMatrix A`,
+    `h.toGraph` is the simple graph whose adjacency matrix is `A`. -/
+@[simps]
+def toGraph [MulZeroOneClass α] [Nontrivial α] (h : IsAdjMatrix A) : SimpleGraph V where
+  Adj i j := A i j = 1
+  symm i j hij := by simp only; rwa [h.symm.apply i j]
+  loopless i := by simp [h]
+#align matrix.is_adj_matrix.to_graph Matrix.IsAdjMatrix.toGraph
+
+instance [MulZeroOneClass α] [Nontrivial α] [DecidableEq α] (h : IsAdjMatrix A) :
+    DecidableRel h.toGraph.Adj := by
+  simp only [toGraph]
+  infer_instance
+
+end IsAdjMatrix
+
+/-- For `A : Matrix V V α`, `A.compl` is supposed to be the adjacency matrix of
+    the complement graph of the graph induced by `A.adjMatrix`. -/
+def compl [Zero α] [One α] [DecidableEq α] [DecidableEq V] (A : Matrix V V α) : Matrix V V α :=
+  fun i j => ite (i = j) 0 (ite (A i j = 0) 1 0)
+#align matrix.compl Matrix.compl
+
+section Compl
+
+variable [DecidableEq α] [DecidableEq V] (A : Matrix V V α)
+
+@[simp]
+theorem compl_apply_diag [Zero α] [One α] (i : V) : A.compl i i = 0 := by simp [compl]
+#align matrix.compl_apply_diag Matrix.compl_apply_diag
+
+@[simp]
+theorem compl_apply [Zero α] [One α] (i j : V) : A.compl i j = 0 ∨ A.compl i j = 1 := by
+  unfold compl
+  split_ifs <;> simp
+#align matrix.compl_apply Matrix.compl_apply
+
+@[simp]
+theorem isSymm_compl [Zero α] [One α] (h : A.IsSymm) : A.compl.IsSymm := by
+  ext
+  simp [compl, h.apply, eq_comm]
+#align matrix.is_symm_compl Matrix.isSymm_compl
+
+@[simp]
+theorem isAdjMatrix_compl [Zero α] [One α] (h : A.IsSymm) : IsAdjMatrix A.compl :=
+  { symm := by simp [h] }
+#align matrix.is_adj_matrix_compl Matrix.isAdjMatrix_compl
+
+namespace IsAdjMatrix
+
+variable {A}
+
+@[simp]
+theorem compl [Zero α] [One α] (h : IsAdjMatrix A) : IsAdjMatrix A.compl :=
+  isAdjMatrix_compl A h.symm
+#align matrix.is_adj_matrix.compl Matrix.IsAdjMatrix.compl
+
+theorem toGraph_compl_eq [MulZeroOneClass α] [Nontrivial α] (h : IsAdjMatrix A) :
+    h.compl.toGraph = h.toGraphᶜ := by
+  ext (v w)
+  cases' h.zero_or_one v w with h h <;> by_cases hvw : v = w <;> simp [Matrix.compl, h, hvw]
+#align matrix.is_adj_matrix.to_graph_compl_eq Matrix.IsAdjMatrix.toGraph_compl_eq
+
+end IsAdjMatrix
+
+end Compl
+
+end Matrix
+
+open Matrix
+
+namespace SimpleGraph
+
+variable (G : SimpleGraph V) [DecidableRel G.Adj]
+
+variable (α)
+
+/-- `adjMatrix G α` is the matrix `A` such that `A i j = (1 : α)` if `i` and `j` are
+  adjacent in the simple graph `G`, and otherwise `A i j = 0`. -/
+def adjMatrix [Zero α] [One α] : Matrix V V α :=
+  of fun i j => if G.Adj i j then (1 : α) else 0
+#align simple_graph.adj_matrix SimpleGraph.adjMatrix
+
+variable {α}
+
+-- TODO: set as an equation lemma for `adjMatrix`, see mathlib4#3024
+@[simp]
+theorem adjMatrix_apply (v w : V) [Zero α] [One α] :
+    G.adjMatrix α v w = if G.Adj v w then 1 else 0 :=
+  rfl
+#align simple_graph.adj_matrix_apply SimpleGraph.adjMatrix_apply
+
+@[simp]
+theorem transpose_adjMatrix [Zero α] [One α] : (G.adjMatrix α)ᵀ = G.adjMatrix α := by
+  ext
+  simp [adj_comm]
+#align simple_graph.transpose_adj_matrix SimpleGraph.transpose_adjMatrix
+
+@[simp]
+theorem isSymm_adjMatrix [Zero α] [One α] : (G.adjMatrix α).IsSymm :=
+  transpose_adjMatrix G
+#align simple_graph.is_symm_adj_matrix SimpleGraph.isSymm_adjMatrix
+
+variable (α)
+
+/-- The adjacency matrix of `G` is an adjacency matrix. -/
+@[simp]
+theorem isAdjMatrix_adjMatrix [Zero α] [One α] : (G.adjMatrix α).IsAdjMatrix :=
+  { zero_or_one := fun i j => by by_cases G.Adj i j <;> simp [h] }
+#align simple_graph.is_adj_matrix_adj_matrix SimpleGraph.isAdjMatrix_adjMatrix
+
+/-- The graph induced by the adjacency matrix of `G` is `G` itself. -/
+theorem toGraph_adjMatrix_eq [MulZeroOneClass α] [Nontrivial α] :
+    (G.isAdjMatrix_adjMatrix α).toGraph = G := by
+  ext
+  simp only [IsAdjMatrix.toGraph_Adj, adjMatrix_apply, ite_eq_left_iff, zero_ne_one]
+  apply Classical.not_not
+#align simple_graph.to_graph_adj_matrix_eq SimpleGraph.toGraph_adjMatrix_eq
+
+variable {α} [Fintype V]
+
+@[simp]
+theorem adjMatrix_dotProduct [NonAssocSemiring α] (v : V) (vec : V → α) :
+    dotProduct (G.adjMatrix α v) vec = ∑ u in G.neighborFinset v, vec u := by
+  simp [neighborFinset_eq_filter, dotProduct, sum_filter]
+#align simple_graph.adj_matrix_dot_product SimpleGraph.adjMatrix_dotProduct
+
+@[simp]
+theorem dotProduct_adjMatrix [NonAssocSemiring α] (v : V) (vec : V → α) :
+    dotProduct vec (G.adjMatrix α v) = ∑ u in G.neighborFinset v, vec u := by
+  simp [neighborFinset_eq_filter, dotProduct, sum_filter, Finset.sum_apply]
+#align simple_graph.dot_product_adj_matrix SimpleGraph.dotProduct_adjMatrix
+
+@[simp]
+theorem adjMatrix_mulVec_apply [NonAssocSemiring α] (v : V) (vec : V → α) :
+    ((G.adjMatrix α).mulVec vec) v = ∑ u in G.neighborFinset v, vec u := by
+  rw [mulVec, adjMatrix_dotProduct]
+#align simple_graph.adj_matrix_mul_vec_apply SimpleGraph.adjMatrix_mulVec_apply
+
+@[simp]
+theorem adjMatrix_vecMul_apply [NonAssocSemiring α] (v : V) (vec : V → α) :
+    ((G.adjMatrix α).vecMul vec) v = ∑ u in G.neighborFinset v, vec u := by
+  simp only [← dotProduct_adjMatrix, vecMul]
+  refine' congr rfl _; ext x
+  rw [← transpose_apply (adjMatrix α G) x v, transpose_adjMatrix]
+#align simple_graph.adj_matrix_vec_mul_apply SimpleGraph.adjMatrix_vecMul_apply
+
+@[simp]
+theorem adjMatrix_mul_apply [NonAssocSemiring α] (M : Matrix V V α) (v w : V) :
+    (G.adjMatrix α ⬝ M) v w = ∑ u in G.neighborFinset v, M u w := by
+  simp [mul_apply, neighborFinset_eq_filter, sum_filter]
+#align simple_graph.adj_matrix_mul_apply SimpleGraph.adjMatrix_mul_apply
+
+@[simp]
+theorem mul_adjMatrix_apply [NonAssocSemiring α] (M : Matrix V V α) (v w : V) :
+    (M ⬝ G.adjMatrix α) v w = ∑ u in G.neighborFinset w, M v u := by
+  simp [mul_apply, neighborFinset_eq_filter, sum_filter, adj_comm]
+#align simple_graph.mul_adj_matrix_apply SimpleGraph.mul_adjMatrix_apply
+
+variable (α)
+
+@[simp]
+theorem trace_adjMatrix [AddCommMonoid α] [One α] : Matrix.trace (G.adjMatrix α) = 0 := by
+  simp [Matrix.trace]
+#align simple_graph.trace_adj_matrix SimpleGraph.trace_adjMatrix
+
+variable {α}
+
+theorem adjMatrix_mul_self_apply_self [NonAssocSemiring α] (i : V) :
+    (G.adjMatrix α ⬝ G.adjMatrix α) i i = degree G i := by simp [degree]
+#align simple_graph.adj_matrix_mul_self_apply_self SimpleGraph.adjMatrix_mul_self_apply_self
+
+variable {G}
+
+-- @[simp] -- Porting note: simp can prove this
+theorem adjMatrix_mulVec_const_apply [Semiring α] {a : α} {v : V} :
+    (G.adjMatrix α).mulVec (Function.const _ a) v = G.degree v * a := by simp [degree]
+#align simple_graph.adj_matrix_mul_vec_const_apply SimpleGraph.adjMatrix_mulVec_const_apply
+
+theorem adjMatrix_mulVec_const_apply_of_regular [Semiring α] {d : ℕ} {a : α}
+    (hd : G.IsRegularOfDegree d) {v : V} : (G.adjMatrix α).mulVec (Function.const _ a) v = d * a :=
+  by simp [hd v]
+#align simple_graph.adj_matrix_mul_vec_const_apply_of_regular SimpleGraph.adjMatrix_mulVec_const_apply_of_regular
+
+theorem adjMatrix_pow_apply_eq_card_walk [DecidableEq V] [Semiring α] (n : ℕ) (u v : V) :
+    (G.adjMatrix α ^ n) u v = Fintype.card { p : G.Walk u v | p.length = n } := by
+  rw [card_set_walk_length_eq]
+  induction' n with n ih generalizing u v
+  · obtain rfl | h := eq_or_ne u v <;> simp [finsetWalkLength, *]
+  · nth_rw 1 [Nat.succ_eq_one_add]
+    simp only [pow_add, pow_one, finsetWalkLength, ih, mul_eq_mul, adjMatrix_mul_apply]
+    rw [Finset.card_bunionᵢ]
+    · norm_cast
+      simp only [Nat.cast_sum, card_map, neighborFinset_def]
+      apply Finset.sum_toFinset_eq_subtype
+    -- Disjointness for card_bUnion
+    · rintro ⟨x, hx⟩ - ⟨y, hy⟩ - hxy
+      rw [disjoint_iff_inf_le]
+      intro p hp
+      simp only [inf_eq_inter, mem_inter, mem_map, Function.Embedding.coeFn_mk, exists_prop] at hp;
+        obtain ⟨⟨px, _, rfl⟩, ⟨py, hpy, hp⟩⟩ := hp
+      cases hp
+      simp at hxy
+#align simple_graph.adj_matrix_pow_apply_eq_card_walk SimpleGraph.adjMatrix_pow_apply_eq_card_walk
+
+end SimpleGraph
+
+namespace Matrix.IsAdjMatrix
+
+variable [MulZeroOneClass α] [Nontrivial α]
+
+variable {A : Matrix V V α} (h : IsAdjMatrix A)
+
+/-- If `A` is qualified as an adjacency matrix,
+    then the adjacency matrix of the graph induced by `A` is itself. -/
+theorem adjMatrix_toGraph_eq [DecidableEq α] : h.toGraph.adjMatrix α = A := by
+  ext (i j)
+  obtain h' | h' := h.zero_or_one i j <;> simp [h']
+#align matrix.is_adj_matrix.adj_matrix_to_graph_eq Matrix.IsAdjMatrix.adjMatrix_toGraph_eq
+
+end Matrix.IsAdjMatrix