Theiss (spelled “Thiess” for the well‑known company) is majority‑owned by CIMIC Group Limited, which currently holds a 60% stake, with the remaining 40% owned by funds advised by Elliott Advisors (UK) Ltd. Thiess itself is an international mining services company based in Brisbane, Australia, and has been part of the wider CIMIC Group structure since it was acquired (via predecessor Leighton Holdings) in the 1980s.

If you meant a different “Theiss” (for example, Dr. Theiss Naturwaren, a German family‑owned cosmetics and pharma company still managed by its founder Dr. Peter Theiss), then that business is owned and run as a family company rather than by a large listed group.

Because “Theiss” can refer to several entities, a more precise name (like “Thiess mining” or “Dr. Theiss Naturwaren”) will help narrow down the exact owner you’re asking about.