Old Faithful is a cone geyser located in Wyoming, in Yellowstone National Park in the United States. Old Faithful was named in 1870 during the Washburn-Langford-Doane Expedition and was the first geyser in the park to receive a name. It is also called the most predictable geographical feature on Earth erupting almost every 91 minutes. The geyser, as well as the nearby Old Faithful Inn, is part of the Old Faithful Historic District.

Eruptions can shoot 3,700 to 8,400 US gallons of boiling water to a height of 106 to 185 feet, lasting from 1.5 to 5 minutes. The average height of an eruption is 145 feet. The highest recorded eruption was 185 feet. Intervals between eruptions can range from 45 to 125 minutes, with an average of 90 minutes apart today.

Old Faithful is not the tallest or largest geyser in the park; that title belongs to the less predictable Steamboat Geyser. Its popularity is more likely due to the small eruption window and high frequency.

After Old Faithful erupted, the two RV Gypsies went to explore other parts of the park (that's when they got caught in a sudden, surprise HAIL storm - use the link at the bottom of this page). They returned later to Old Faithful just as it was finishing another eruption.