diff --git a/include/zephyr/arch/riscv/structs.h b/include/zephyr/arch/riscv/structs.h
index 11e4935a51283..691b67a922a0a 100644
--- a/include/zephyr/arch/riscv/structs.h
+++ b/include/zephyr/arch/riscv/structs.h
@@ -22,6 +22,15 @@ struct _cpu_arch {
 	atomic_ptr_val_t fpu_owner;
 	uint32_t fpu_state;
 #endif
+#if defined(CONFIG_CPP) && !defined(CONFIG_USERSPACE) &&                                           \
+	!(defined(CONFIG_SMP) || (CONFIG_MP_MAX_NUM_CPUS > 1)) && !defined(CONFIG_FPU_SHARING)
+	/* Empty struct has size 0 in C, size 1 in C++. Force them to be the same. */
+	uint8_t unused_cpp_size_compatibility;
+#endif
 };
 
+#if defined(CONFIG_CPP)
+BUILD_ASSERT(sizeof(struct _cpu_arch) >= 1);
+#endif
+
 #endif /* ZEPHYR_INCLUDE_RISCV_STRUCTS_H_ */
diff --git a/include/zephyr/arch/riscv/thread.h b/include/zephyr/arch/riscv/thread.h
index 08f14b61cebf7..54765550415d6 100644
--- a/include/zephyr/arch/riscv/thread.h
+++ b/include/zephyr/arch/riscv/thread.h
@@ -84,8 +84,17 @@ struct _thread_arch {
 	unsigned long m_mode_pmpaddr_regs[PMP_M_MODE_SLOTS];
 	unsigned long m_mode_pmpcfg_regs[PMP_M_MODE_SLOTS / sizeof(unsigned long)];
 #endif
+#if defined(CONFIG_CPP) && !defined(CONFIG_FPU_SHARING) && !defined(CONFIG_USERSPACE) &&           \
+	!defined(CONFIG_PMP_STACK_GUARD)
+	/* Empty struct has size 0 in C, size 1 in C++. Force them to be the same. */
+	uint8_t unused_cpp_size_compatibility;
+#endif
 };
 
+#if defined(CONFIG_CPP)
+BUILD_ASSERT(sizeof(struct _thread_arch) >= 1);
+#endif
+
 typedef struct _thread_arch _thread_arch_t;
 
 #endif /* _ASMLANGUAGE */