Johann Ernst of Nassau-Weilburg (Weilburg, 13 June 1664 – Heidelberg, 27 February 1719) was an Imperial Generalfeldmarschall, from 1675 to 1688 Count and from 1688 until his death Prince (Fürst) of Nassau-Weiburg.

Johann Ernst became the only ruler of Nassau-Weilburg when his brother Frederick William Louis was killed in 1684 during the siege of Buda.

When Prince Eugene of Savoy marched in 1703 to Bavaria against the French Marshal Villars, Johann Ernst stayed on the Rhine to defend Landau against Tallard.

8000 men were lost and Johann Ernst's eldest son, Colonel Frederick Ludwig of Nassau-Weilburg was killed.

In the 1700s, Johann Ernst initiated the expansion of his residence, Schloss Weilburg, from a Renaissance Palace to a Baroque complex, adding orangeries, stables, a church and terrace gardens.