D环（D-loop），或称置换环（displacement loop）在分子生物学中是指一个双链DNA的两条单链互相分离，并被另外一条（第三条）核酸链隔开的结构。第三条核酸链与与之配对的核酸链具有互补的碱基串行。D环区域形成了三链DNA结构。正如插图所示，因为这种结构类似于大写的「D」（被置换的那条DNA链像「）」而新形成的双链像「|」）而得名「D环」。

在不少过程中，D环均会出现，比如DNA修复。另外，在线粒体DNA中（作为一种半稳定的结构）以及端粒中，亦存在D环结构。

In molecular biology, a displacement loop or D-loop is a DNA structure where the two strands of a double-stranded DNA molecule are separated for a stretch and held apart by a third strand of DNA. The third strand has a base sequence which is complementary to one of the main strands and pairs with it, thus displacing the other complementary main strand in the region. Within that region the structure is thus a form of triple-stranded DNA. A diagram in the paper introducing the term illustrated the D-loop with a shape resembling a capital "D", where the displaced strand formed the loop of the "D".