<p>Natural rubber, with annual production of 15 million tonnes, is the most used bio-elastomer. Improving its resistance to crack growth is highly desired, to prolong its service life for many applications and eventually improve its sustainability. Here we markedly amplify the resistance to crack growth in natural rubber by forming a tanglemer, a polymer network in which entanglements greatly outnumber crosslinks. Specifically, we cast natural rubber latex without high-intensity processing that cuts long polymers. The long polymers densely entangle by thermal motion and are then sparsely crosslinked. At a crack tip, long polymer strands between neighbouring crosslinks deconcentrate stress, extend strain-induced crystallization over a large region and enhance crystallinity. For example, when the ratio of crosslinks to repeat units reduces from 10⁻² to 10⁻³, the network amplifies fatigue threshold from ~50 J m⁻² to ~200 J m⁻², and toughness from ~10⁴ J m⁻² to over 10⁵ J m⁻². Overall, this work provides a viable strategy to improve the practical applicability of natural rubber, contributing to the development of sustainable polymers.</p>