The Social Investment Agency (SIA; Māori: Toi Hau Tāngata) is a New Zealand government department that leads the implementation of the government's investment approach to social spending.

The concept of "social investment" describes the use of data to target spending on social services towards certain disadvantaged groups with the aim of achieving the best "return" in terms of long-term social outcomes and government costs resulting from social issues.

Originally run out of a unit within the Ministry of Social Development, the Social Investment Agency was first established as a departmental agency hosted by the State Services Commission on 1 July 2017. It was later known as the Social Wellbeing Agency (2020–2024) and reverted to its original name when it was re-established as a standalone central agency.

The Social Investment Agency is responsible for overseeing, coordinating and implementing the government's social investment approach. Its functions include:

The responsible minister is the Minister for Social Investment, Nicola Willis. The agency's chief executive position is vacant after Andrew Coster resigned in December 2025. The agency is headed by a board known as the Social Investment Board, which oversees its program. The board members include former Secretary to the Treasury Graham Scott, former New Zealand Labour Party president and former Howard League chief executive Mike Williams, and former Minister for Māori Development Te Ururoa Flavell.

The agency defines its social investment approach as using data and evidence to understand people's needs; establishing clear, measurable goals and focusing on effective strategies; measuring and comparing service effectiveness using a informed decision-making approach; and empowering local providers to deliver services to their communities. The agency's work will also be supported by a Social Investment Fund, expected to come into operation by 2025.

The following ministers have held the office of Minister for Social Investment.

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