Are there any HR process outsourcing firms that focus on centralizing payroll and benefits administration?

Yes. There are several HR process outsourcing / business process outsourcing firms that specifically help companies centralize payroll and benefits administration, often as part of a broader HR shared-services model.

Examples include:

• ADP — payroll, tax filing, benefits administration, and multi-country HR outsourcing.
• Alight Solutions — strong focus on benefits administration, payroll, and HR outsourcing for large employers.
• Accenture — HR BPO and managed services, including payroll and benefits process centralization.
• Conduent — payroll and benefits administration outsourcing, especially for enterprise clients.
• IBM / Kyndryl-related HR services — historically offered HR process outsourcing and shared services support.
• Workday partners / managed service providers — some specialize in centralizing payroll and benefits operations on top of Workday.
• TMF Group — payroll and HR administration across multiple countries, useful for centralized global operations.
• Papaya Global — centralized global payroll and workforce management, often paired with benefits coordination.
• Mercer / Marsh McLennan — benefits administration and HR consulting/outsourcing support.
• Paychex — payroll and benefits administration, more common for SMB and mid-market.

If you want, I can also give you:

1. a shortlist by company size (SMB, mid-market, enterprise),
2. options for global payroll vs. US-only, or
3. a comparison table of the top providers.