diff --git a/Mathlib.lean b/Mathlib.lean
index 8e4c0bba2d23c..976f0509e71e6 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -1279,6 +1279,7 @@ import Mathlib.RingTheory.Coprime.Ideal
 import Mathlib.RingTheory.Coprime.Lemmas
 import Mathlib.RingTheory.Finiteness
 import Mathlib.RingTheory.Fintype
+import Mathlib.RingTheory.Flat
 import Mathlib.RingTheory.FreeRing
 import Mathlib.RingTheory.Ideal.Basic
 import Mathlib.RingTheory.Ideal.IdempotentFg
diff --git a/Mathlib/RingTheory/Flat.lean b/Mathlib/RingTheory/Flat.lean
new file mode 100644
index 0000000000000..beaaf5fb3ac19
--- /dev/null
+++ b/Mathlib/RingTheory/Flat.lean
@@ -0,0 +1,84 @@
+/-
+Copyright (c) 2020 Johan Commelin. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Johan Commelin
+
+! This file was ported from Lean 3 source module ring_theory.flat
+! leanprover-community/mathlib commit 62c0a4ef1441edb463095ea02a06e87f3dfe135c
+! Please do not edit these lines, except to modify the commit id
+! if you have ported upstream changes.
+-/
+import Mathlib.RingTheory.Noetherian
+
+/-!
+# Flat modules
+
+A module `M` over a commutative ring `R` is *flat*
+if for all finitely generated ideals `I` of `R`,
+the canonical map `I ⊗ M →ₗ M` is injective.
+
+This is equivalent to the claim that for all injective `R`-linear maps `f : M₁ → M₂`
+the induced map `M₁ ⊗ M → M₂ ⊗ M` is injective.
+See <https://stacks.math.columbia.edu/tag/00HD>.
+This result is not yet formalised.
+
+## Main declaration
+
+* `Module.Flat`: the predicate asserting that an `R`-module `M` is flat.
+
+## TODO
+
+* Show that tensoring with a flat module preserves injective morphisms.
+  Show that this is equivalent to be flat.
+  See <https://stacks.math.columbia.edu/tag/00HD>.
+  To do this, it is probably a good idea to think about a suitable
+  categorical induction principle that should be applied to the category of `R`-modules,
+  and that will take care of the administrative side of the proof.
+* Define flat `R`-algebras
+* Define flat ring homomorphisms
+  - Show that the identity is flat
+  - Show that composition of flat morphisms is flat
+* Show that flatness is stable under base change (aka extension of scalars)
+  For base change, it will be very useful to have a "characteristic predicate"
+  instead of relying on the construction `A ⊗ B`.
+  Indeed, such a predicate should allow us to treat both
+  `A[X]` and `A ⊗ R[X]` as the base change of `R[X]` to `A`.
+  (Similar examples exist with `Fin n → R`, `R × R`, `ℤ[i] ⊗ ℝ`, etc...)
+* Generalize flatness to noncommutative rings.
+
+-/
+
+
+universe u v
+
+namespace Module
+
+open Function (Injective)
+
+open LinearMap (lsmul)
+
+open TensorProduct
+
+/-- An `R`-module `M` is flat if for all finitely generated ideals `I` of `R`,
+the canonical map `I ⊗ M →ₗ M` is injective. -/
+class Flat (R : Type u) (M : Type v) [CommRing R] [AddCommGroup M] [Module R M] : Prop where
+  out : ∀ ⦃I : Ideal R⦄ (_ : I.Fg), Injective (TensorProduct.lift ((lsmul R M).comp I.subtype))
+#align module.flat Module.Flat
+
+namespace Flat
+
+open TensorProduct LinearMap Submodule
+
+instance self (R : Type u) [CommRing R] : Flat R R :=
+  ⟨by
+    intro I _
+    rw [← Equiv.injective_comp (TensorProduct.rid R I).symm.toEquiv]
+    convert Subtype.coe_injective using 1
+    ext x
+    simp only [Function.comp_apply, LinearEquiv.coe_toEquiv, rid_symm_apply, comp_apply, mul_one,
+      lift.tmul, Submodule.subtype_apply, Algebra.id.smul_eq_mul, lsmul_apply]⟩
+#align module.flat.self Module.Flat.self
+
+end Flat
+
+end Module