Keepers had hoped the big cats would breed but have been left devastated after Asim, the male they brought in, attacked Melati.
An endangered Sumatran tiger was mauled to death at London Zoo on Friday, just moments after it was introduced to a potential mate for the first time.
But tensions "quickly escalated", things became "more aggressive" and Melati died in a fight, the zoo said.
After spending time apart in the tiger enclosure to get used to the new arrangement, the two were then introduced to each other earlier.
"This morning, the two tigers were in separate paddocks and the adjoining door was opened to allow them to meet".
The zoo says keepers intervened with loud noises, flares and alarms but were too late to save Melati.
When they finally got Asim away, Melati was dead.
Staff are "heartbroken by this turn of events", the zoo said.
But they cautioned meetings between big cats, however carefully planned, are "always considered to be high risk".
This image taken from video and made available by London Zoo, shows Sumatran tiger named Asim at London Zoo, Wednesday Jan. 30, 2019.
Before the fatal interaction, Asim, 7, and Melati, 10, were able to see, smell, and react to each other through their separate enclosures - a way for zookeepers to monitor their behaviors towards each other to ensure safety for both animals, the statement said.
Sumatran tigers are described as critically endangered by the World Wildlife Federation, which estimages there are fewer than 400 in the wild.
"Tiger Territory will remain closed for the rest of the day while our team focus on caring for Asim".
After Asim - whose name, ironically, means "protector" in Arabic - arrived at the zoo last month, head tiger keeper Kathryn Sanders described him as "a handsome, confident cat who is known for being very affectionate with the ladies in his life".
ZSL London Zoo coordinates a European breeding program and a global species management program.