前置码（英语：Prefix code），又译前缀码，是一种编码系统。这种编码系统通常是可变长度码，在其中的每个码字，都具备「前置性质」（prefix property），也就是说，在编码中的每个码字，都不需要其他码字当成前置部位。举例而言，编码字 {9, 55} 具备了前置性质，但编码字{9, 5, 59, 55}就不具备，因为其中的"5"，是"59"及"55"的前置字。这也被称为无首码的代码（prefix-free codes，PFC，无前缀码）。

A prefix code is a type of code system (typically a variable-length code) distinguished by its possession of the "prefix property", which requires that there is no code word in the system that is a prefix (initial segment) of any other code word in the system. For example, a code with code words {9, 55} has the prefix property; a code consisting of {9, 5, 59, 55} does not, because "5" is a prefix of "59" and also of "55". A prefix code is a uniquely decodable code: a receiver can identify each word without requiring a special marker between words.