A trench knife is a combat knife designed to kill or incapacitate an enemy at close quarters, such as in a trench or other confined area. It was developed as a close combat weapon for soldiers attacking enemy trenches during the First World War. An example of a World War I trench knife is the German Army's Nahkampfmesser (close combat knife).

During the Second World War, the trench knife, by this time also called a combat knife, was developed into new designs. On the German side, the Nahkampfmesser and associated knives were widely issued to the ordinary soldier for combat and utility purposes, while Allied armies mostly issued trench knives to elite infantry units and soldiers not equipped with the bayonet.

With the exception of the German Nahkampfmesser (or close combat knife), most early trench knives were fabricated by hand by individual soldiers or blacksmiths for the purpose of silently killing sentries and other soldiers during trench raids. These early "trench knives" were often shortened and sharpened Army-issue bayonets. One type of stabbing weapon, the French nail, was made by cutting and pointing the steel stakes used to support the barbed wire protecting trenches. Some historians say that some trench knives models were inspired by the Bowie knife.

Soon afterwards, these fabricated trench knives were used in defensive close-quarters trench warfare, and such fighting soon revealed limitations in existing designs.

A more elegant form of the French Nail was the Poignard-Baïonnette Lebel M1886/14. Approved as a standard military infantry weapon after its development by Lieutenant Colonel Coutrot of the French Army, the Poignard-Baïonnette Lebel consisted of a long, needle-pointed, stiletto-profile blade with wooden handle and an integrated knuckle guard made of steel. Originally a conversion of the French Épée-Baïonnette Modèle 1886 (bayonet), and designed strictly as an offensive weapon, the Poignard-Baïonnette Lebel used a section of the M1886 Lebel's long, narrow stiletto-type cruciform blade, designed to quickly kill an enemy soldier with a deep thrust. Up to three trench knives could be constructed from a single M1886 Lebel bayonet.

Because French industry was working under wartime conditions with numerous material shortages, often using subcontracted labor, even officially sanctioned French Army trench knives tend to vary significantly from knife to knife. The need for knives was so great that already-understrength French Army formations were forced to demobilize hundreds of former cutlery workers so that they could return to their former jobs and begin quantity production of trench knives for the armed forces. As the war went on, newer and more versatile blade-type trench knife patterns such as the double-edged dagger Couteau Poignard Mle 1916 dit Le Vengeur began to replace the French Nail and earlier stiletto-style trench knives. The French lead in trench knife development was closely followed by the United States, which introduced three successive trench knife models - the M1917, M1918, and Mark I (1918) - all based directly or indirectly upon previous French designs.

The German Nahkampfmesser (translated: close combat knife) was the standard issue German combat knife during the First World War. It remained in service in modified form through the end of the Second World War. Most of these knives had slab wooden grips and metal sheaths and were sturdily made. According to one authoritative source, German-issued trench knives of World War I were "conventional, general-purpose, cut-and-thrust knives" with blades that were "for the most part approximately six inches in length, single-edged with a top leading false edge ... although double-edged blades are occasionally encountered."

After the French Nail was employed against them, German forces on the Western Front also began to employ converted steel barbed-wire stakes as stabbing weapons for use by their own soldiers.