Ange Postecoglou's start was little better, his Al-Nassr team featuring Cristiano Ronaldo losing 4-0 to Al-Ain of the United Arab Emirates.
Aleksandar Cavric scored a first-half hat-trick for Kashima, title winners in 2018, as the J-League champions dished out a lesson to Mark Milligan's side at Kashima Stadium in the eastern league phase opener.
It was the Jets' first Asian Champions League tie since being beaten 4-1 by the same opponents in the 2018 playoffs.
"I think we started the game very well, we pressed them almost all the time, so it was quite intense in the first half," said Cavric, who scored in the 11th and 16th minutes to give Kashima early control on Tuesday.
"We didn't give them much time to think, so that's why we were able to create many chances."
James Delianov's 19th-minute own goal left Newcastle with a challenging task that became insurmountable when Homare Tokuda hit the fourth in the 40th minute, before Cavric completed his hat-trick two minutes later.
Tokuda and Kyosuke Tagawa scored in the second half for Kashima, before Patrick Wood netted a 79th-minute consolation for the visitors.
The Jets qualified as the A-League Men regular-season premiers. They are Australia's only representatives after Adelaide United were knocked out in the preliminary round.
Newcastle's next match is at home to Gamba Osaka, who began with a 4-1 win over Adelaide's conquerors, Cong An Ha Noi from Vietnam.
Saudi Arabian champions Al-Nassr, who have never won the competition, conceded two goals to Moroccan forward Houssine Rahimi and one to his brother Soufiane Rahimi, who then set up Georgios Giakoumakis for a fourth.
The 41-year-old Ronaldo, looking for career goal No. 980, had a free kick late in the game, but it was saved by goalkeeper Khalid Eisa.
"The players gave everything they had, and I do not place any responsibility on them … and the responsibility is my responsibility," Postecoglou said. "Al-Nassr as a whole was poor."
Elsewhere, Kevin Muscat's Shanghai Port survived a frantic finish to secure a 6-4 win over Ratchaburi.
Port were in charge after Prince Ampem scored three times in 15 minutes late in the first half, with Wu Lei adding a fourth in the 51st minute.
Madagascar international Njiva Rakotoharimalala scored a late hat-trick for the Thai hosts, but Port held on to secure their first win in the competition since November 2024.
There were also wins for South Korea's Daejeon Citizen and Beijing Guoan, while Neil Etheridge made an 87th-minute penalty save to ensure 10-man Buriram United of Thailand secured a 1-1 draw with hosts Johor Darul Ta'zim in Malaysia.
Four-time champions Al-Hilal beat Qatar's Al-Gharafa 2-1.
- with AP
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