磨刀石，又称砥砺、砺石，是一种工具。刀具经使用后刀锋易变钝，需经常在磨刀石上来回拉动磨利。其材质分有金刚石、氧化锆等等。 磨刀也属于一种技术，没有经过一定的练习，易使金属部份过度的消耗且达不到良好的锋利度。 磨刀前需先确认磨刀石为平面，因磨刀石是属于消耗品，固经常所使用的部份会凹陷，需先整面后才使用以得到预期效果。 陶瓷刀是不可以使用传统磨刀石打磨的刀类，需使用特定的机器或材料，否则将会毁损。

《山海经·西山经》已经有磨刀石的记载了。磨刀石，旧称砥砺，细者为砥，粗者为砺。磨刀石又可分为页岩、粉砂岩、砂岩等。松花石是一种磨刀石，满清八旗入关前常以松花石制成磨刀石。

Sharpening stones, water stones or whetstones are used to grind and hone the edges of steel tools and implements.

Examples of items that may be sharpened with a sharpening stone include scissors, scythes, knives, razors and tools such as chisels, hand scrapers and plane blades. Though it is sometimes mistaken as a reference to the water often used to lubricate such stones, the word "whetstone" is a compound word formed with the word "whet", which means to sharpen a blade, not the word "wet". The process of using a sharpening stone is called stoning.