向斜（Syncline）是一种地层排列方式，指岩层发生折曲时，其形状向下凹陷者。在一般平地上，向斜的地层上半部受到侵蚀变平，会形成中间较新，两侧较古老的地层排列方式。

向斜地区的岩石在褶皱作用下被挤压，比较坚硬，抗侵蚀能力较强。

向斜是良好的储水构造。石油、天然气、地下水三者比较，天然气的密度最小，石油次之，水的密度最大，且向斜的岩层向下弯曲，即储水。相反，背斜是良好的储油构造。

因为向斜岩层向下弯曲，受力集中于中心。所以，同一平面上各点受力不均匀，不宜修建铁路、隧道等工程。

In structural geology, a syncline is a fold with younger layers closer to the center of the structure. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline (i.e. a trough); but synclines that point upwards, or perched, can be found when strata have been overturned and folded (an antiformal syncline).