Castlecliff is a suburb of Whanganui, in the Whanganui District and Manawatū-Whanganui region of New Zealand's North Island. The name was given by the Harbour Board, on the suggestion of the future Prime Minister, John Ballance, when it established the township on what were described as "barren sandhills" in 1882. Many of the streets were named after Harbour Board members. The northern harbour breakwater extended 900 ft (270 m) by 1885. The freezing works was built in 1891.

Castlecliff covers 8.65 km² (3.34 sq mi) and had an estimated population of 4,260 as of June 2025, with a population density of 492 people per km².

Castlecliff had a population of 3,630 at the 2018 New Zealand census, an increase of 444 people (13.9%) since the 2013 census, and an increase of 210 people (6.1%) since the 2006 census. There were 1,341 households, comprising 1,830 males and 1,797 females, giving a sex ratio of 1.02 males per female, with 861 people (23.7%) aged under 15 years, 678 (18.7%) aged 15 to 29, 1,584 (43.6%) aged 30 to 64, and 510 (14.0%) aged 65 or older.

Ethnicities were 70.0% European/Pākehā, 42.4% Māori, 4.7% Pacific peoples, 2.6% Asian, and 1.8% other ethnicities. People may identify with more than one ethnicity.

The percentage of people born overseas was 7.9, compared with 27.1% nationally.

Although some people chose not to answer the census's question about religious affiliation, 54.1% had no religion, 28.1% were Christian, 6.4% had Māori religious beliefs, 0.4% were Hindu, 0.5% were Buddhist and 2.4% had other religions.

Of those at least 15 years old, 234 (8.5%) people had a bachelor's or higher degree, and 792 (28.6%) people had no formal qualifications. 123 people (4.4%) earned over $70,000 compared to 17.2% nationally. The employment status of those at least 15 was that 1,119 (40.4%) people were employed full-time, 369 (13.3%) were part-time, and 189 (6.8%) were unemployed.

Castlecliff School is a state primary school for Year 1 to 6 students, with a roll of 165 as of July 2025.