The supermarket took 600g packs of Tesco Simply Roast Meatloaf, which are supposed to contain 35% beef and 22% pork, from its shelves.
The batch was made between October last year and January at Eurostock in Craigavon, Northern Ireland.
A spokesman said it was the fourth of its products to test positive for horsemeat contamination and apologised.
The list of tainted food includes Tesco's own-brand hamburgers, bolognese and lasagne, found to contain up to 100% horse.
Tim J Smith, its group technical director, said: "Tests on 15 other lines from the same manufacturing site were clear. We will complete our investigation before deciding whether to continue using the supplier.
"We are very sorry we have had a further product which has failed to meet the high standards we and our customers expect."
He said the company had now tested 500 product lines.
Last week Tesco vowed to improve its supply chain after the scandal as bosses try to repair any damage to its reputation.
Further revelations are expected in the next few weeks as the Food Standards Agency said last Friday that seven recent samples, representing six products, came back positive for more than 1% horse DNA.
Nearly a fortnight ago the retailer began an apologetic "What burgers have taught us" advertising campaign in an attempt to regain public and British farmers' confidence.
Many of the UK's biggest food firms and supermarkets have recalled beef products since January after tests found they contained horse DNA.
Early tests found horse DNA in products such as Aldi's special frozen beef lasagne and special frozen spaghetti bolognese, Co-op frozen quarter-pounder burgers, Findus beef lasagne and Tesco Value frozen burgers and spaghetti bolognese.
A second wave of tests revealed contamination of Asda's chilled beef bolognese sauce and beefburgers; minced beef and halal minced beef sold by Sodexo, which supplies food to schools, care homes and the armed forces and a Whitbread Group lasagne and beefburger.
The third round of tests affected Birds Eye Traditional Spaghetti Bolognese and Beef Lasagne, Taco Bell's ground beef and Brakes' spicy minced beef skewer.
Last month, a frozen burger found in the kitchen of Cumbernauld High School, North Lanarkshire, contained horse DNA. North Lanarkshire Council said the burger was supplied by Brakes Group.
The discovery led to councils across Scotland being advised to place a hold on frozen burgers.
Furniture retailer Ikea also withdrew wiener sausages in the UK last month after tests found "indications" of horsemeat, and it also withdrew a batch of its traditional meatballs.
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