diff --git a/src/frontend/decoder/thumb1.h b/src/frontend/decoder/thumb1.h
index 3c92cb22..bffc64d6 100644
--- a/src/frontend/decoder/thumb1.h
+++ b/src/frontend/decoder/thumb1.h
@@ -56,7 +56,7 @@ private:
 };
 
 template <typename V>
-static const std::array<Thumb1Matcher<V>, 22> g_thumb1_instruction_table {{
+static const std::array<Thumb1Matcher<V>, 23> g_thumb1_instruction_table {{
 
 #define INST(fn, name, bitstring) detail::detail<Thumb1Matcher, u16, 16>::GetMatcher<decltype(fn), fn>(name, bitstring)
 
@@ -83,7 +83,7 @@ static const std::array<Thumb1Matcher<V>, 22> g_thumb1_instruction_table {{
     { INST(&V::thumb1_SBC_reg,        "SBC (reg)",                "0100000110mmmddd") },
     { INST(&V::thumb1_ROR_reg,        "ROR (reg)",                "0100000111sssddd") },
     { INST(&V::thumb1_TST_reg,        "TST (reg)",                "0100001000mmmnnn") },
-    //{ INST(&V::thumb1_NEGS_rr,        "NEGS (rr)",                "0100001001mmmddd") },
+    { INST(&V::thumb1_RSB_imm,        "RSB (imm)",                "0100001001nnnddd") },
     //{ INST(&V::thumb1_CMP_rr,         "CMP (rr)",                 "0100001010mmmnnn") },
     //{ INST(&V::thumb1_CMN_rr,         "CMN (rr)",                 "0100001011mmmnnn") },
     //{ INST(&V::thumb1_ORRS_rr,        "ORRS (rr)",                "0100001100mmmddd") },
diff --git a/src/frontend/disassembler_thumb.cpp b/src/frontend/disassembler_thumb.cpp
index 8a8703d7..82e141db 100644
--- a/src/frontend/disassembler_thumb.cpp
+++ b/src/frontend/disassembler_thumb.cpp
@@ -182,6 +182,11 @@ public:
         return Common::StringFromFormat("tst %s, %s", RegStr(n), RegStr(m));
     }
 
+    std::string thumb1_RSB_imm(Reg n, Reg d) {
+        // Pre-UAL syntax: NEGS <Rd>, <Rn>
+        return Common::StringFromFormat("rsbs %s, %s, #0", RegStr(d), RegStr(n));
+    }
+
     std::string thumb1_ADD_reg_t2(bool d_n_hi, Reg m, Reg d_n_lo) {
         Reg d_n = d_n_hi ? (d_n_lo + 8) : d_n_lo;
         return Common::StringFromFormat("add %s, %s", RegStr(d_n), RegStr(m));
diff --git a/src/frontend/translate_thumb.cpp b/src/frontend/translate_thumb.cpp
index 8c9111a9..8641c575 100644
--- a/src/frontend/translate_thumb.cpp
+++ b/src/frontend/translate_thumb.cpp
@@ -258,6 +258,17 @@ struct TranslatorVisitor final {
         ir.SetZFlag(ir.IsZero(result));
         return true;
     }
+    bool thumb1_RSB_imm(Reg n, Reg d) {
+        // RSBS <Rd>, <Rn>, #0
+        // Rd can never encode R15.
+        auto result = ir.SubWithCarry(ir.Imm32(0), ir.GetRegister(n), ir.Imm1(1));
+        ir.SetRegister(d, result.result);
+        ir.SetNFlag(ir.MostSignificantBit(result.result));
+        ir.SetZFlag(ir.IsZero(result.result));
+        ir.SetCFlag(result.carry);
+        ir.SetVFlag(result.overflow);
+        return true;
+    }
 
     bool thumb1_ADD_reg_t2(bool d_n_hi, Reg m, Reg d_n_lo) {
         Reg d_n = d_n_hi ? (d_n_lo + 8) : d_n_lo;