diff --git a/Mathlib.lean b/Mathlib.lean
index bb572a5a061e4f..cdd90a93d8d70a 100644
--- a/Mathlib.lean
+++ b/Mathlib.lean
@@ -6151,6 +6151,7 @@ public import Mathlib.Probability.Combinatorics.BinomialRandomGraph.Defs
 public import Mathlib.Probability.CondVar
 public import Mathlib.Probability.ConditionalExpectation
 public import Mathlib.Probability.ConditionalProbability
+public import Mathlib.Probability.Decision.HypothesisTesting.NeymanPearson
 public import Mathlib.Probability.Decision.Risk.Basic
 public import Mathlib.Probability.Decision.Risk.Defs
 public import Mathlib.Probability.Density
diff --git a/Mathlib/Probability/Decision/HypothesisTesting/NeymanPearson.lean b/Mathlib/Probability/Decision/HypothesisTesting/NeymanPearson.lean
new file mode 100644
index 00000000000000..a45aabe12f63fa
--- /dev/null
+++ b/Mathlib/Probability/Decision/HypothesisTesting/NeymanPearson.lean
@@ -0,0 +1,178 @@
+/-
+Copyright (c) 2026 Zixiao Wang. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Zixiao Wang, Rajarshi Mukherjee
+-/
+module
+
+public import Mathlib.MeasureTheory.Measure.Decomposition.Lebesgue
+public import Mathlib.MeasureTheory.Measure.Decomposition.RadonNikodym
+
+/-!
+# The Neyman–Pearson lemma (sum-of-errors form)
+
+For two finite measures `P` and `Q` on a measurable space `Ω`, the Neyman–Pearson lemma
+characterizes the randomized tests `T : Ω → ℝ` with values in `[0, 1]` that minimize
+the sum of error masses
+`∫ T dP + t · ∫ (1 - T) dQ`
+where `t > 0` is a fixed cost weight on Type II errors (equivalently, the test compares
+the likelihood ratio against the threshold `t⁻¹`).
+
+## Main definitions
+
+* `ProbabilityTheory.NeymanPearson.npLikelihoodRatio P Q` — the likelihood ratio
+  `(dQ/d(P+Q)) / (dP/d(P+Q))` of `Q` against `P`.
+* `ProbabilityTheory.NeymanPearson.npTest P Q t η` — the Neyman–Pearson test:
+`1` if the likelihood ratio is `> t⁻¹`, `η` if it equals `t⁻¹`, and `0` otherwise;
+  forced to `1` on `{dP/d(P+Q) = 0}`.
+* `ProbabilityTheory.NeymanPearson.npSumError P Q t T` — the sum-of-errors cost
+  `∫ T dP + t · ∫ (1 - T) dQ`.
+
+## Main result
+
+* `ProbabilityTheory.NeymanPearson.neyman_pearson` — `npTest P Q t η` minimizes
+  `npSumError P Q t` over the set of measurable `[0, 1]`-valued tests, for any
+  `t > 0` and `0 ≤ η ≤ 1`.
+
+## References
+* Ingster, Y. and Suslina, I.A., 2012. Nonparametric goodness-of-fit testing under Gaussian models (Vol. 169). Springer Science & Business Media.
+-/
+
+@[expose] public section
+
+open MeasureTheory
+open scoped ENNReal
+
+namespace ProbabilityTheory.NeymanPearson
+
+variable {Ω : Type*} [MeasurableSpace Ω] (P Q : Measure Ω)
+
+/-- The likelihood ratio of `Q` against `P` with respect to the dominating measure
+`P + Q`, i.e. `(dQ/d(P+Q)) / (dP/d(P+Q))`. On the `P`-null set where `dP/d(P+Q)`
+vanishes, this evaluates to `0` by Lean's `a / 0 = 0` convention; the value there
+is immaterial to the optimality statement `neyman_pearson` because `npTest` is
+hard-coded to `1` on that set. -/
+noncomputable def npLikelihoodRatio : Ω → ℝ := fun ω ↦
+  (Q.rnDeriv (P + Q) ω).toReal / (P.rnDeriv (P + Q) ω).toReal
+
+/-- The Neyman–Pearson randomized test against the threshold `t⁻¹`: returns `1` if the
+likelihood ratio is `> t⁻¹`, `η` at the boundary `= t⁻¹`, and `0` otherwise; returns `1`
+on the `P`-null set where the ratio is undefined. The test is optimal for
+`npSumError P Q t` when `t > 0`. Boundedness in `[0, 1]` requires `0 ≤ η ≤ 1`
+(see `npTest_nonneg` and `npTest_le_one`). -/
+noncomputable def npTest (t η : ℝ) : Ω → ℝ := fun ω ↦
+  if (P.rnDeriv (P + Q) ω).toReal = 0 then 1
+  else if npLikelihoodRatio P Q ω > t⁻¹ then 1
+  else if npLikelihoodRatio P Q ω = t⁻¹ then η
+  else 0
+
+/-- The sum-of-errors cost functional `∫ T dP + t · ∫ (1 - T) dQ`. -/
+noncomputable def npSumError (t : ℝ) (T : Ω → ℝ) : ℝ :=
+  ∫ ω, T ω ∂P + t * ∫ ω, (1 - T ω) ∂Q
+
+/-- The likelihood ratio is measurable. -/
+@[fun_prop]
+lemma measurable_npLikelihoodRatio : Measurable (npLikelihoodRatio P Q) :=
+  (Measure.measurable_rnDeriv Q (P + Q)).ennreal_toReal.div
+    (Measure.measurable_rnDeriv P (P + Q)).ennreal_toReal
+
+/-- The Neyman–Pearson test is measurable. -/
+@[fun_prop]
+lemma measurable_npTest (t η : ℝ) : Measurable (npTest P Q t η) := by
+  have hh := measurable_npLikelihoodRatio P Q
+  have hp : Measurable (fun ω ↦ (P.rnDeriv (P + Q) ω).toReal) := by fun_prop
+  exact Measurable.ite (measurableSet_eq_fun hp measurable_const) measurable_const <|
+    Measurable.ite (measurableSet_lt measurable_const hh) measurable_const <|
+      Measurable.ite (measurableSet_eq_fun hh measurable_const) measurable_const measurable_const
+
+/-- The Neyman–Pearson test is non-negative when `0 ≤ η`. -/
+lemma npTest_nonneg {t η : ℝ} (hη0 : 0 ≤ η) (ω : Ω) : 0 ≤ npTest P Q t η ω := by
+  simp only [npTest]
+  split_ifs <;> linarith
+
+/-- The Neyman–Pearson test is bounded by `1` when `η ≤ 1`. -/
+lemma npTest_le_one {t η : ℝ} (hη1 : η ≤ 1) (ω : Ω) : npTest P Q t η ω ≤ 1 := by
+  simp only [npTest]
+  split_ifs <;> linarith
+
+/-- **Neyman–Pearson lemma (sum-of-errors form).** For finite measures `P` and `Q` on
+a measurable space `Ω`, the randomized test `npTest P Q t η` minimizes
+`npSumError P Q t T = ∫ T dP + t · ∫ (1 - T) dQ` over all measurable `[0, 1]`-valued
+tests `T`, for any threshold `t > 0` and any `0 ≤ η ≤ 1`. -/
+theorem neyman_pearson
+    [IsFiniteMeasure P] [IsFiniteMeasure Q]
+    (t η : ℝ) (ht : 0 < t) (hη : η ∈ Set.Icc (0 : ℝ) 1) :
+    IsMinOn (npSumError P Q t)
+      { T : Ω → ℝ | Measurable T ∧ (∀ ω, 0 ≤ T ω) ∧ (∀ ω, T ω ≤ 1) }
+      (npTest P Q t η) := by
+  intro T hT
+  obtain ⟨hTm, hT0, hT1⟩ := hT
+  obtain ⟨hη0, hη1⟩ := hη
+  set τ := npTest P Q t η
+  change npSumError P Q t τ ≤ npSumError P Q t T
+  have hPμ : P ≪ P + Q := Measure.AbsolutelyContinuous.add_right .rfl Q
+  have hQμ : Q ≪ P + Q := Measure.AbsolutelyContinuous.add_right' .rfl P
+  have hτm : Measurable τ := measurable_npTest P Q t η
+  have hτ0 : ∀ ω, 0 ≤ τ ω := npTest_nonneg P Q hη0
+  have hτ1 : ∀ ω, τ ω ≤ 1 := npTest_le_one P Q hη1
+  have h_int_μ : ∀ F : Ω → ℝ, Measurable F → (∀ ω, 0 ≤ F ω) → (∀ ω, F ω ≤ 1)
+      → Integrable (fun ω ↦ (P.rnDeriv (P + Q) ω).toReal * F ω) (P + Q) ∧
+        Integrable (fun ω ↦ (Q.rnDeriv (P + Q) ω).toReal * (1 - F ω)) (P + Q) := by
+    intro F hFm hF0 hF1
+    have hF_P : Integrable F P := .of_mem_Icc 0 1 hFm.aemeasurable
+      (ae_of_all _ fun ω ↦ Set.mem_Icc.mpr ⟨hF0 ω, hF1 ω⟩)
+    have h1F_m : Measurable (fun ω ↦ 1 - F ω) := by fun_prop
+    have h1F_Q : Integrable (fun ω ↦ 1 - F ω) Q := .of_mem_Icc 0 1 h1F_m.aemeasurable
+      (ae_of_all _ fun ω ↦ Set.mem_Icc.mpr ⟨by linarith [hF1 ω], by linarith [hF0 ω]⟩)
+    exact ⟨(integrable_toReal_rnDeriv_mul_iff hPμ).mpr hF_P,
+           (integrable_toReal_rnDeriv_mul_iff hQμ).mpr h1F_Q⟩
+  obtain ⟨hpτ, hq1τ⟩ := h_int_μ τ hτm hτ0 hτ1
+  obtain ⟨hpT, hq1T⟩ := h_int_μ T hTm hT0 hT1
+  have h_change : ∀ F : Ω → ℝ,
+      Integrable (fun ω ↦ (P.rnDeriv (P + Q) ω).toReal * F ω) (P + Q) →
+      Integrable (fun ω ↦ (Q.rnDeriv (P + Q) ω).toReal * (1 - F ω)) (P + Q) →
+      npSumError P Q t F =
+        ∫ ω, (P.rnDeriv (P + Q) ω).toReal * F ω +
+          t * ((Q.rnDeriv (P + Q) ω).toReal * (1 - F ω)) ∂(P + Q) := fun F hpF hq1F ↦ by
+    change ∫ ω, F ω ∂P + t * ∫ ω, (1 - F ω) ∂Q = _
+    rw [← integral_toReal_rnDeriv_mul hPμ, ← integral_toReal_rnDeriv_mul hQμ,
+        ← integral_const_mul, ← integral_add hpF (hq1F.const_mul t)]
+  rw [h_change τ hpτ hq1τ, h_change T hpT hq1T]
+  refine integral_mono (hpτ.add (hq1τ.const_mul t)) (hpT.add (hq1T.const_mul t)) ?_
+  intro ω
+  dsimp only
+  by_cases hp_zero : (P.rnDeriv (P + Q) ω).toReal = 0
+  · have hτ_one : τ ω = 1 := by
+      change npTest P Q t η ω = 1
+      simp only [npTest, if_pos hp_zero]
+    rw [hp_zero, hτ_one]
+    nlinarith [mul_nonneg ht.le (mul_nonneg
+      (ENNReal.toReal_nonneg : (0:ℝ) ≤ (Q.rnDeriv (P + Q) ω).toReal)
+      (sub_nonneg.mpr (hT1 ω)))]
+  · have hp_pos : 0 < (P.rnDeriv (P + Q) ω).toReal :=
+      lt_of_le_of_ne ENNReal.toReal_nonneg (Ne.symm hp_zero)
+    have h_ratio : npLikelihoodRatio P Q ω =
+        (Q.rnDeriv (P + Q) ω).toReal / (P.rnDeriv (P + Q) ω).toReal := rfl
+    rcases lt_trichotomy (npLikelihoodRatio P Q ω) t⁻¹ with h_lt | h_eq | h_gt
+    · have hτ_zero : τ ω = 0 := by
+        change npTest P Q t η ω = 0
+        simp only [npTest, if_neg hp_zero, if_neg (not_lt.mpr h_lt.le), if_neg (ne_of_lt h_lt)]
+      rw [h_ratio, div_lt_iff₀ hp_pos] at h_lt
+      have htq_lt_p : t * (Q.rnDeriv (P + Q) ω).toReal < (P.rnDeriv (P + Q) ω).toReal := by
+        have := mul_lt_mul_of_pos_left h_lt ht
+        rwa [← mul_assoc, mul_inv_cancel₀ ht.ne', one_mul] at this
+      rw [hτ_zero]; nlinarith [hT0 ω, hT1 ω, htq_lt_p]
+    · rw [h_ratio, div_eq_iff hp_zero] at h_eq
+      have hp_eq : (P.rnDeriv (P + Q) ω).toReal = t * (Q.rnDeriv (P + Q) ω).toReal := by
+        rw [h_eq, ← mul_assoc, mul_inv_cancel₀ ht.ne', one_mul]
+      rw [hp_eq]; ring_nf; exact le_rfl
+    · have hτ_one : τ ω = 1 := by
+        change npTest P Q t η ω = 1
+        simp only [npTest, if_neg hp_zero, if_pos h_gt]
+      rw [h_ratio, lt_div_iff₀ hp_pos] at h_gt
+      have hp_lt_tq : (P.rnDeriv (P + Q) ω).toReal < t * (Q.rnDeriv (P + Q) ω).toReal := by
+        have := mul_lt_mul_of_pos_left h_gt ht
+        rwa [← mul_assoc, mul_inv_cancel₀ ht.ne', one_mul] at this
+      rw [hτ_one]; nlinarith [hT0 ω, hT1 ω, hp_lt_tq]
+
+end ProbabilityTheory.NeymanPearson