First performed by Herb Alpert (1968).
First released (as “That Guy’s in Love”) by Danny Williams (1968).
Hit versions by Herb Alpert (US #1/MOR #1/CAN #1/UK #3/AUS #1 1968), Dionne Warwick (as “This Girl’s in Love with You” US #7/MOR #2/R&B #7/CAN #7 1969).
Also recorded by Burt Bacharach (1969)
From the wiki:”‘This Guy’s in Love with You’ was written by Burt Bacharach and Hal David. [T]he original performance originated when Herb Alpert, while visiting at Bacharach’s office, asked, ‘Say, Burt, do you happen to have any old compositions lying around that you and Hal never recorded; maybe one I might be able to use?’ Alpert said he made it his practice to ask songwriters that particular question: often a ‘lost pearl’ was revealed. As it happened, Bacharach recalled one, found the lyrics and score sheet in his office filing cabinet, and offered it to Alpert: ‘Here, Herb … you might like this one.’
“Alpert first sang ‘This Guy’s in Love with You’ on his April 1968 television special, The Beat of the Brass. In response to numerous viewer telephone calls to the network following the broadcast, Alpert decided that the song should be recorded and used as the promotional single for the subsequent May 1968 release of the TV special’s soundtrack. But, the first release of ‘This Guy’s in Love with You’, titled ‘That Guy’s in Love’, was in the UK by South African-born singer Danny Williams in late April 1968, for his self-titled album. Williams’ recording, however, was not released as a single.