Gregorio di Montelongo (or da Monte Longo; c. 1200 – 1269) was the Bishop of Tripoli from 1249 until 1251 and the Patriarch of Aquileia from 1251 until his death.
Gregorio was born in Ferentino into a family related to the counts of Segni, original of Montelongo in Molise.
He also led the anti-Hohenstaufen party in the region when Frederick's heir, Conrad IV of Germany, incited the Istrian communes to revolt.
Gregorio also participated in the "crusade" against Ezzelino III da Romano, leading personally the defense of Padua in September 1255 and sending troops to the Guelph faction until 1257.
Previously, Gregorio had signed a treaty with the Republic of Venice which set the respective areas of influence in the contented Istrian region.