复杂指令集（英文：Complex Instruction Set Computing；缩写：CISC）是一种微处理器指令集架构，每个指令可运行若干低级操作，诸如从内存读取、保存、和计算操作，全部集於单一指令之中。与之相对的是精简指令集。

复杂指令集的特点是指令数目多而复杂，每条指令字长并不相等，电脑必须加以判读，并为此付出了性能的代价。

在精简指令集处理器发迹以前，许多电脑架构尝试跨越「语义鸿沟」──设计出借由提供「高端」指令支持高端编程语言的指令集，诸如进程调用和返回，循环指令诸如「若非零则减量和分支」和复杂寻址模式以允许数据结构和数组访问以结合至单一指令。与复杂指令集相比，精简指令集实现更容易，指令并行执行程度更好，编译器的效率更高。

Complex instruction set computing (CISC /ˈsɪsk/) is a processor design where single instructions can execute several low-level operations (such as a load from memory, an arithmetic operation, and a memory store) or are capable of multi-step operations or addressing modes within single instructions. The term was retroactively coined in contrast to reduced instruction set computer (RISC) and has therefore become something of an umbrella term for everything that is not RISC, i.e. everything from large and complex mainframes to simplistic microcontrollers where memory load and store operations are not separated from arithmetic instructions.