diff --git a/src/components/features/StrategyResults.tsx b/src/components/features/StrategyResults.tsx
index de7dc50..698c27c 100644
--- a/src/components/features/StrategyResults.tsx
+++ b/src/components/features/StrategyResults.tsx
@@ -257,7 +257,7 @@ const StrategyResults: React.FC<StrategyResultsProps> = ({ result, profile, isDa
           <ShieldAlert className="w-4 h-4 text-amber-500" />
           <h4 className="text-sm font-bold">Tax Calculation Breakdown</h4>
         </div>
-        <div className="grid grid-cols-1 sm:grid-cols-2 md:grid-cols-4 gap-4 text-xs">
+        <div className="grid grid-cols-1 sm:grid-cols-2 md:grid-cols-5 gap-4 text-xs">
           <div className="p-2 bg-slate-50 dark:bg-slate-800 rounded">
             <span className="text-slate-500">Standard Deduction</span>
             <p className="font-bold text-slate-900 dark:text-white">{formatCurrency(result.standardDeduction)}</p>
@@ -274,6 +274,10 @@ const StrategyResults: React.FC<StrategyResultsProps> = ({ result, profile, isDa
             <span className="text-slate-500">Mandatory RMD</span>
             <p className="font-bold text-slate-900 dark:text-white">{formatCurrency(result.rmdAmount)}</p>
           </div>
+          <div className="p-2 bg-slate-50 dark:bg-slate-800 rounded">
+            <span className="text-slate-500 flex items-center gap-1">NIIT (3.8%) <Tooltip content="Net Investment Income Tax: 3.8% surtax on investment income (dividends, capital gains) when MAGI exceeds $200K (Single) / $250K (MFJ). IRA distributions increase MAGI but are not subject to NIIT themselves." /></span>
+            <p className="font-bold text-slate-900 dark:text-white">{formatCurrency(result.niitAmount)}</p>
+          </div>
         </div>
       </div>
 
diff --git a/src/components/features/TaxReference.tsx b/src/components/features/TaxReference.tsx
index 98778de..8554eba 100644
--- a/src/components/features/TaxReference.tsx
+++ b/src/components/features/TaxReference.tsx
@@ -665,7 +665,7 @@ const TaxReference: React.FC = () => {
                             <h4 className="font-semibold text-slate-800 dark:text-white mb-2">Limitations</h4>
                             <ul className="text-sm text-slate-600 dark:text-slate-400 space-y-1">
                                 <li>• State taxes are not included</li>
-                                <li>• Net Investment Income Tax (NIIT) not modeled</li>
+                                <li>• Net Investment Income Tax (NIIT) modeled at 3.8% on investment income above MAGI thresholds</li>
                                 <li>• Tax brackets are estimates and may change</li>
                                 <li>• Does not account for itemized deductions</li>
                                 <li>• Social Security COLA adjustments assumed 0%</li>
diff --git a/src/constants/index.ts b/src/constants/index.ts
index a297d6f..b8a4b7f 100644
--- a/src/constants/index.ts
+++ b/src/constants/index.ts
@@ -179,3 +179,12 @@ export const SINGLE_LIFE_EXPECTANCY_TABLE: Record<number, number> = {
   70: 18.8, 71: 18.0, 72: 17.2, 73: 16.4, 74: 15.6,
   75: 14.8,
 };
+
+// Net Investment Income Tax (NIIT) - IRC §1411
+// 3.8% on the lesser of: net investment income OR MAGI exceeding threshold
+// Thresholds are NOT indexed for inflation (static since 2013)
+export const NIIT_RATE = 0.038;
+export const NIIT_THRESHOLDS = {
+  [FilingStatus.Single]: 200000,
+  [FilingStatus.MarriedJoint]: 250000,
+};
diff --git a/src/services/calculationEngine.ts b/src/services/calculationEngine.ts
index adb27fa..419da0f 100644
--- a/src/services/calculationEngine.ts
+++ b/src/services/calculationEngine.ts
@@ -2,7 +2,8 @@ import { UserProfile, StrategyResult, WithdrawalSource, FilingStatus, LongevityR
 import {
   STANDARD_DEDUCTION, AGE_DEDUCTION, TAX_BRACKETS, CAP_GAINS_BRACKETS, UNIFORM_LIFETIME_TABLE, RMD_START_AGE,
   SENIOR_DEDUCTION, SENIOR_DEDUCTION_PHASEOUT, SS_TAX_THRESHOLDS, IRMAA_THRESHOLDS, IRMAA_SAFETY_BUFFER,
-  FED_MIDTERM_RATE_120, SINGLE_LIFE_EXPECTANCY_TABLE // Imported
+  FED_MIDTERM_RATE_120, SINGLE_LIFE_EXPECTANCY_TABLE, // Imported
+  NIIT_RATE, NIIT_THRESHOLDS
 } from '../constants';
 
 /**
@@ -26,6 +27,24 @@ const calculateTaxableSocialSecurity = (ssAmount: number, otherIncome: number, f
   );
 };
 
+/**
+ * Calculates Net Investment Income Tax (NIIT) - IRC §1411
+ * 3.8% on the LESSER of:
+ *   (a) Net investment income (dividends, capital gains from brokerage)
+ *   (b) MAGI exceeding threshold ($200K Single, $250K MFJ)
+ * IRA distributions count toward MAGI but NOT net investment income.
+ */
+const calculateNIIT = (
+  netInvestmentIncome: number,
+  magi: number,
+  filingStatus: FilingStatus
+): number => {
+  const threshold = NIIT_THRESHOLDS[filingStatus];
+  const magiExcess = Math.max(0, magi - threshold);
+  if (magiExcess <= 0 || netInvestmentIncome <= 0) return 0;
+  return NIIT_RATE * Math.min(netInvestmentIncome, magiExcess);
+};
+
 /**
  * Calculates Federal Tax using the 'Two-Layer Cake' approach.
  */
@@ -288,6 +307,7 @@ export const calculateStrategy = (profile: UserProfile): StrategyResult => {
     let grossCash = totalAnnualSS + income.pension + brokerageDividends;
     let ordIncomeForTax = income.pension + ordinaryDividends;
     let capGainsForTax = qualifiedDividends;
+    let netInvestmentIncome = brokerageDividends; // Dividends are always NII; brokerage gains added below
     let currentPenalty = 0;
 
     // 0. RMDs (Always First)
@@ -356,7 +376,10 @@ export const calculateStrategy = (profile: UserProfile): StrategyResult => {
         // Add to Cash / Tax / Penalty
         grossCash += pull;
         if (step.taxType === 'Ordinary') ordIncomeForTax += taxableAmt;
-        if (step.taxType === 'CapitalGains') capGainsForTax += taxableAmt;
+        if (step.taxType === 'CapitalGains') {
+          capGainsForTax += taxableAmt;
+          netInvestmentIncome += taxableAmt; // Brokerage gains are NII
+        }
 
         if (step.penalty) {
           currentPenalty += pull * 0.10;
@@ -370,7 +393,12 @@ export const calculateStrategy = (profile: UserProfile): StrategyResult => {
     liquidityGapWarning = gap > 1 || currentPenalty > 0;
 
     const finalTaxableSS = calculateTaxableSocialSecurity(totalAnnualSS, ordIncomeForTax, filingStatus);
-    const iterationTax = calculateFederalTax(ordIncomeForTax + finalTaxableSS, capGainsForTax, filingStatus, standardDeduction);
+    const iterationIncomeTax = calculateFederalTax(ordIncomeForTax + finalTaxableSS, capGainsForTax, filingStatus, standardDeduction);
+
+    // NIIT: MAGI = all income before standard deduction
+    const magi = ordIncomeForTax + finalTaxableSS + capGainsForTax;
+    const niitTax = calculateNIIT(netInvestmentIncome, magi, filingStatus);
+    const iterationTax = iterationIncomeTax + niitTax;
 
     lastResult = {
       totalWithdrawal: grossCash,
@@ -384,6 +412,7 @@ export const calculateStrategy = (profile: UserProfile): StrategyResult => {
       taxableSocialSecurity: finalTaxableSS,
       currentYearSocialSecurity: totalAnnualSS,
       provisionalIncome: ordIncomeForTax + (0.5 * totalAnnualSS),
+      niitAmount: niitTax,
       standardDeduction,
       notes: currentPenalty > 0 ? [`Includes $${currentPenalty.toFixed(0)} early withdrawal penalty.`] : [],
       nominalSpendingNeeded
@@ -601,7 +630,11 @@ export const calculateLongevity = (profile: UserProfile, strategy: StrategyResul
     const stdDeduction = STANDARD_DEDUCTION[profile.filingStatus] +
       calculateAgeDeduction(age, profile.filingStatus, spouseAgeThisYear);
 
-    const estimatedTax = calculateFederalTax(totalOrdinaryForTax + taxableSS, totalCapGainsForTax, profile.filingStatus, stdDeduction);
+    const incomeTax = calculateFederalTax(totalOrdinaryForTax + taxableSS, totalCapGainsForTax, profile.filingStatus, stdDeduction);
+    const longevityNII = currentDividends + brokerageGain;
+    const longevityMAGI = totalOrdinaryForTax + taxableSS + totalCapGainsForTax;
+    const niitTax = calculateNIIT(longevityNII, longevityMAGI, profile.filingStatus);
+    const estimatedTax = incomeTax + niitTax;
     const totalCashFlow = fromBrokerage + fromTrad + fromRoth + fromHSA + totalFixedIncome;
     const effectiveTaxRate = totalCashFlow > 0 ? estimatedTax / totalCashFlow : 0;
 
diff --git a/src/types/core.ts b/src/types/core.ts
index 07f06d2..c58cc97 100644
--- a/src/types/core.ts
+++ b/src/types/core.ts
@@ -75,6 +75,7 @@ export interface StrategyResult {
   taxableSocialSecurity: number;
   currentYearSocialSecurity: number;
   provisionalIncome: number;
+  niitAmount: number; // Net Investment Income Tax (3.8% surtax)
   standardDeduction: number;
   notes: string[];
   nominalSpendingNeeded: number; // Adjusted for inflation if needed