Hat Yai (Thai: หาดใหญ่, pronounced [hàːt jàj], also Haad Yai or Had Yai) is a city in southern Thailand near the Malaysian border and the fifth-largest city in Thailand with a population of 191,696 (2024) in the city municipality (thesaban nakhon / city proper) itself and an urban population of 406,513 (2024) in the entire district of Amphoe Hat Yai.

Hat Yai is the largest city of Southern Thailand and is part of the Hat Yai-Songkhla Metropolitan Area (with a population of about 800,000) which include 2 big districts (Hat Yai, Mueang Songkhla) and 4 small neighboring districts (Bang Klam, Khlong Hoi Khong, Na Mom, Singhanakhon), forming the largest metropolitan area in the south, and the fourth-largest metropolitan area of the country. The city is often mistaken for being the provincial capital. In fact, Songkhla is the capital and the center of administration and culture while Hat Yai is the business center.

Originally named Khok Samet Chun (โคกเสม็ดชุน, "shore eugenia knoll"), Hat Yai was a small village until the southern railway was built there, making it one of the major rail hubs of the time. The junction which connected the town of Songkhla with the main route was formerly in the U Tapao area, but was later moved to Khok Samet Chun in 1922 when the U Tapao area turned out to be flood-prone. At that time, Khok Samet Chun had only four residents, but thanks to the investments of Khun Niphat Chinnakorn (the railway contractor for the railway line from Nakhon Si Thammarat to Pattani) it quickly grew into a small town.

In 1928, Hat Yai was made a community (chumchon), which was upgraded to a sanitary district (sukhaphiban) on December 11, 1935. It covered an area of 4.4 km² (1.7 sq mi), and was administered by the first mayor, Udom Bunlong. In 1938, the municipal administration building was completed. On March 16, 1949, Hat Yai was granted town status (thesaban mueang). On May 10, 1961, the area covered by the municipality was increased to 8 km² (3.1 sq mi). As a result of the town's continuing growth, on August 13, 1968, a larger, new municipal administration building was opened. On April 24, 1977, the total area of the municipality was enlarged for the second time to 21 km² (8.1 sq mi). Finally, in 1995, the town was upgraded to city status (thesaban nakhon). There are a total of 102 communities (chumchon), divided into 4 zones.

The name "Hat Yai" is a short version of "mahat yai", meaning big mahat (Thai: มะหาด) tree, a relative of jackfruits in genus Artocarpus.

Hat Yai Junction has become the transportation hub of southern Thailand since the railway was constructed. One of the largest railway stations in the south, Hat Yai Railway Station is an international railway station which handles 28 passenger trains per day, including 26 trains served by State Railway of Thailand and 2 trains served by KTMB of Malaysia. Hat Yai is also the hub for local train services in southern Thailand.

Parallel to the railway line is the Asian highway 2. Asian highway 18 originates in Hat Yai and runs south along the east coast of the Malay peninsula.

Hat Yai Bus Terminal is a major transport hub in southern Thailand. It offers bus services that link Hat Yai with nearly every town and city in the southern region as well as other destinations, including Bangkok and Nakhon Ratchasima. Hat Yai also has a minibus terminal which hosts the largest minibus service in southern Thailand. There are also several private minibus services which focus on tourist destinations, including Phuket, Ko Samui, Ko Pha Ngan, Pak Bara Peer in Satun (a gateway to the islands in the southern Andaman Sea), Langkawi, Penang, Kuala Lumpur, and Singapore. The minibus service has gained popularity in recent years as minibuses are considered a faster way to travel.