The 2017 Atlantic hurricane season was a hyperactive tropical cyclone season and the costliest on record, with a damage total of at least $294.92 billion (USD). Featuring 17 named storms, 10 hurricanes, and 6 major hurricanes, 2017 had the fifth-most named storms since reliable records began in 1851 – tied with 1936 – with 6 major hurricanes. Most of the season's damage was due to hurricanes Harvey, Irma, and Maria. Another notable hurricane, Nate, was the worst natural disaster in Costa Rican history. The names Harvey, Irma, Maria, and Nate were retired following the season due to the number of deaths and amount of damage they caused. Collectively, the tropical cyclones were responsible for at least 3,364 deaths – the most fatalities in a single season since 2005 as well as the highest accumulated cyclone energy (ACE), while a record three hurricanes each had an ACE of over 40 : Irma, Jose, and Maria.
This season is also one of only seven years on record to feature multiple Category 5 hurricanes and the only season other than 2007 with two hurricanes making landfall at that intensity. All ten of the season's hurricanes occurred in a row – the greatest number of consecutive hurricanes in the satellite era, and tied for the highest number of consecutive hurricanes ever observed in the Atlantic basin.
The season officially began on June 1 and ended on November 30. These dates historically describe the period of year when most tropical cyclones form in the Atlantic basin and are adopted by convention. However, as shown by Tropical Storm Arlene in April, the formation of tropical cyclones was possible at other times of the year. In late August, Hurricane Harvey struck Texas and became the first major hurricane to make landfall in the United States since Wilma in 2005 and the strongest since Charley in 2004. The storm tied the record for the costliest tropical cyclone and broke the record for most rainfall dropped by a tropical cyclone in the United States, with extreme flooding in the Houston area. In early September, Hurricane Irma became the first Category 5 hurricane to impact the northern Leeward Islands on record, later making landfall in the Florida Keys as a Category 4 hurricane. In terms of sustained winds, Irma, at the time, became the strongest hurricane ever recorded in the Atlantic basin outside of the Gulf of Mexico and Caribbean Sea, with maximum sustained winds of 180 mph (285 km/h); it was later surpassed in 2019 by Hurricane Dorian. In mid September, Hurricane Maria became the first Category 5 hurricane in history to strike the island of Dominica. It later made landfall in Puerto Rico as a high-end Category 4 hurricane with catastrophic effect. Most of the deaths from this season occurred from Maria. In early October, Hurricane Nate became the fastest-moving tropical cyclone in the Gulf of Mexico on record and the fourth hurricane to strike the contiguous United States in 2017. Slightly over a week later, Hurricane Ophelia became the easternmost major hurricane in the Atlantic basin on record, and later impacted most of Northern Europe as an extratropical cyclone. The season concluded with Tropical Storm Rina, which became extratropical on November 9.
Initial predictions for the season anticipated that an El Niño would develop, lowering tropical cyclone activity. However, the predicted El Niño failed to develop, with cool-neutral conditions developing instead, later progressing to a La Niña – the second one in a row. This led forecasters to raise their predicted totals, with some later anticipating that the season could be the most active since 2010.
Beginning in 2017, the National Hurricane Center (NHC) had the option to issue advisories – and thus allow watches and warnings to be issued – on disturbances that are not yet tropical cyclones but have a high chance to become one, and are expected to bring tropical storm or hurricane conditions to landmasses within 48 hours. Such systems are termed "Potential Tropical Cyclones". The first storm to receive this designation was Potential Tropical Cyclone Two, which later developed into Tropical Storm Bret, east-southeast of the Windward Islands on June 18. In addition, the numbering that a potential tropical cyclone receives would be retained for the rest of the hurricane season, meaning that the next tropical system would be designated with the following number, even though potential tropical cyclones do not qualify as tropical cyclones. This was first demonstrated with Potential Tropical Cyclone Ten, which failed to develop into a tropical cyclone.