The Aucklander followed her silver in the non-Olympic 1500m with another second place finish behind American star Katie Ledecky, who swam yet another world record.
Ledecky won in 8min 07.39s, chomping 3.71s off her own world mark while Boyle touched in 8:17.65s to claim not only a new national record but also the best Oceania mark.
Britain's Jaz Carlin finished strongly to take third .50s behind Boyle.
It takes to 10 the number of world championship medals won by New Zealand swimmers, dating back to backstroker Gary Hurring's silver over 200m in Berlin in 1978.
"I'm really happy with my performance and my time, it's fantastic to get another medal and really happy with a personal best time for me and a New Zealand record, which is amazing," Boyle said today.
The 27-year-old's plan was to get out ahead of the chasing pack in pursuit of Ledecky, and it worked a treat.
"I knew I had to go out pretty strong to stay ahead of the other girls in the race.
"I was happy I was reasonably close to Ledecky halfway through, then just tried to keep going from there.
"Luckily I had enough left on the last lap but it was quite close so I guess the plan worked out pretty well."
Boyle was aware of action on the other side of the pool, in the form of Carlin, without knowing the identity of the challenger.
''But I knew I was ahead so I just (focussed) on getting my hand to the wall."
Boyle has high praise for the 18-year-old Ledecky, who became the first person to win four freestyle titles from 200m out at the same worlds.
''She's a lovely girl. She's quite funny and really nice to talk to.
''What she's doing for the sport is amazing and I think it helps all swimmers open our minds to what is possible."
Boyle has a sponsors photoshoot in Montenegro early this week, and a World Cup meet in Paris before heading home for a well-deserved break. After that it's back to the Gold Coast and full steam ahead for the Rio Olympics next year.
''I'm grateful for the position I'm now in in terms of my racing and time and will do everything I can over the next year to improve. It's an exciting time."
New Zealand's world championship medals
Silver: Gary Hurring (200m backstroke, Berlin 1978), Anthony Mosse (200m butterfly, Madrid 1986), Danyon Loader (200m butterfly, Rome 1994), Lauren Boyle (800m and 1500m freestyle, Kazan 2015).
Bronze: Loader (200m and 400m freestyle, Rome 1994), Boyle (400m, 800m and 1500m freestyle, Barcelona 2013).
