Tamper-evident envelope element of sheet material, comprising: - at least one rear wall, - at least one first front flap engaged with the rear wall, optionally at a perimetral edge of this latter, - at least one second front flap also engaged with the rear wall, optionally at the perimetral edge thereof, opposite to said first front flap, wherein the first and second front flaps (3, 4) are configured for defining at least one closed condition wherein these latter at least partially overlap each other, wherein the first and second front flaps (3, 4) are further configured for defining at least one open condition, wherein these latter are distanced from each other, said envelope element further comprising at least one safety device of sheet material, said safety device comprising: - at least one first coupling portion borne by the first front flap, - at least one second coupling portion borne by the second front flap, the first and second coupling portions (13, 14) being configured for stably engaging with each other during a first closed condition of the first and second front flaps (3, 4), at least one between the first and second coupling portions (13, 14) exhibiting at least one removable portion configured for separating from at least one between the first and second front flap (3, 4) after a first opening condition thereof subsequent to said first closed condition for preventing a subsequent coupling of said first and second coupling portions (13, 14) and for revealing tampering with the envelope, characterized by the fact that the second coupling portion faces and is at least partially in contact with the second front flap. - at least one rear wall, - at least one first front flap engaged with the rear wall, optionally at a perimetral edge of this latter, - at least one second front flap also engaged with the rear wall, optionally at the perimetral edge thereof, opposite to said first front flap, wherein the first and second front flaps (3, 4) are configured for defining at least one closed condition wherein these latter at least partially overlap each other, wherein the first and second front flaps (3, 4) are further configured for defining at least one open condition, wherein these latter are distanced from each other, said envelope element further comprising at least one safety device of sheet material, said safety device comprising: - at least one first coupling portion borne by the first front flap, - at least one second coupling portion borne by the second front flap, - at least one first coupling portion borne by the first front flap, - at least one second coupling portion borne by the second front flap, the first and second coupling portions (13, 14) being configured for stably engaging with each other during a first closed condition of the first and second front flaps (3, 4), at least one between the first and second coupling portions (13, 14) exhibiting at least one removable portion configured for separating from at least one between the first and second front flap (3, 4) after a first opening condition thereof subsequent to said first closed condition for preventing a subsequent coupling of said first and second coupling portions (13, 14) and for revealing tampering with the envelope, characterized by the fact that the second coupling portion faces and is at least partially in contact with the second front flap.