The coupler is mounted on the end of the boom of a boom-tractor, and provides secure locking-in of a bucket. The coupler having been set to its latched-open condition, the tractor is driven to the bucket so that a bottom-lug of the bucket enters the coupler, whereupon the bucket is picked up and is automatically securely locked into coupler, without any manipulation of the coupler being required of the tractor-driver other than manipulation of the boom. For setting the bucket down on the ground, after use, the driver manually pulls out a slider of the coupler. That action sets the coupler into its slider-held condition, whereby when the tractor is driven away from the bucket, the coupler is automatically left set in the said latched-open condition.