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Using a dataset of public procurement auctions and registered shareholders of all bidding firms in Singapore, we study the effects of ownership networks on prices and efficiency in product markets. A network-based measure of ownership connections shows that participating bidders with common owners or common owners' owners are more likely to submit identical bids, and identical bids are positively correlated with contract prices. Our structural estimates show that removing ownership network effects can improve a contractor's cost efficiency by more than it reduces contract price, highlighting how ownership networks can hinder competition. Our findings are robust to falsification tests, bid rounding, placebo tests using other common stakeholder relationships, and sample weighting based on our machine learning prediction of the auction format.