The Day of the Triffids was cited by Karl Edward Wagner as one of the thirteen best science-fiction horror novels. Arthur C. Clarke called it an "immortal story".
In his book Billion Year Spree: The History of Science Fiction, Brian Aldiss coined the term cosy catastrophe to describe the subgenre of post-war apocalyptic fiction in which society is destroyed save for a handful of survivors, who are able to enjoy a relatively comfortable existence. He specifically singled out The Day of the Triffids as an example of this genre.
Anthony Boucher and J. Francis McComas praised it, saying "rarely have the details of [the] collapse been treated with such detailed plausibility and human immediacy, and never has the collapse been attributed to such an unusual and terrifying source.". Forrest J Ackerman wrote in Astounding Science Fiction that Triffids "is extraordinarily well carried out, with the exception of a somewhat anticlimactic if perhaps inevitable conclusion."
However, Groff Conklin, reviewing the novel's initial book publication, characterised it as "a good run-of-the-mill affair" and "pleasant reading... provided you aren't out hunting science fiction masterpieces."