Liverpool's manager shared the incredible news with the 19-year-old at John Lennon Airport on Wednesday morning as the Reds prepared to depart for a training camp in Marbella.
Alexander-Arnold joins club colleague Jordan Henderson in Gareth Southgate’s 23-man selection for the Three Lions, with Adam Lallana on the standby list.
Speaking to Liverpoolfc.com in Marbella, Klopp detailed the moment he made the young defender aware of the "fantastic" honour.
“It is nice, a nice story," Klopp said. "Gareth Southgate gave me a call this morning and told me that I am allowed to tell him, but then I had some press work to do and didn’t have time.
“So, in the bus to the plane, I saw Trent for the first time. I thought maybe he knew already and someone else had called him. I had the honour of telling him and it was nice to see the reaction in his eyes. It’s fantastic, it’s well deserved, what a season. What a season! Can you be too young? No. You can be not good enough – but he is good enough.
“It’s just a wonderful little piece more which makes this season special, even before the last game [against Real Madrid].
“For Adam, it’s a great thing as well. After that season with only three or four games, I don’t think that any other player would have been in the mind of the manager, but Gareth really likes him.
"He is only on standby, OK, but in his situation that’s the best thing. He can train with us and then go to the national team and train there. Then, if anything happens, Gareth Southgate knows he has a fantastic player on standby.
“Hendo as well, not to forget – our skipper is going to the World Cup and I am happy about that as well!
“All three, well deserved. It’s really nice.”