异染色质（英语：Heterochromatin）是DNA内一种紧密组合的结构，转录作用在其中受到限制。

目前已知染色质有两种类型，分别是真染色质与异染色质。两者差异最早是以受到染色之后的颜色深度来分辨，前者较淡，而后者，也就是异染色质则较深，可见其结构较为紧密（组蛋白包覆率及双螺旋化的程度较较高）。异染色质一般分布于细胞核的边缘地带。

异染色质大多是由不具遗传活性的卫星串行（satellite sequences）所构成。其中的基因皆受到不同程度抑制，有些即使在真染色质内也无法表现。在细胞周期的S phase中，异染色质比真染色质更晚进行复制，且只在真核生物中存在，着丝粒及端粒皆属于异染色质，雌性体内去活化的X染色体（也就是巴尔氏体）也是。

Heterochromatin is a tightly packed form of DNA, which comes in multiple varieties. Because it is tightly packed, it is inaccessible to polymerases and is therefore not transcribed, (or so we thought. According to Volpe et al, and many other papers since, we see that much of this DNA is in fact transcribed, however it is continuously turned over via a RITS pathway) . These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin. Both play a role in the expression of genes.