Nouns ending in -ier are usually masculine.
Is bouclier masculine or feminine?
lebouclier
The rule behind it
95.4% of -ier nouns are masculine
In context
Il s’est acheté un bouclier et une épée.
He bought a shield and a sword.
Je ne dispose pas de bouclier.
I have no shield.
Il me faut un bouclier.
I need a shield.