  Swift J1734.5-3027 is a hard X-ray transient discovered by Swift while
undergoing an outburst in September 2013. Archival observations showed that
this source underwent a previous episode of enhanced X-ray activity in May-June
2013. In this paper we report on the analysis of all X-ray data collected
during the outburst in September 2013, the first that could be intensively
followed-up by several X-ray facilities. Our data-set includes INTEGRAL, Swift,
and XMM-Newton observations. From the timing and spectral analysis of these
observations, we show that a long type-I X-ray burst took place during the
source outburst, making Swift J1734.5-3027 a new member of the class of
bursting neutron star low-mass X-ray binaries. The burst lasted for about 1.9
ks and reached a peak flux of (6.0$\pm$1.8)$\times$10$^{-8}$ erg cm$^{-2}$
s$^{-1}$ in the 0.5-100 keV energy range. The estimated burst fluence in the
same energy range is (1.10$\pm$0.10)$\times$10$^{-5}$ erg cm$^{-2}$. By
assuming that a photospheric radius expansion took place during the first
$\sim$200 s of the burst and that the accreted material was predominantly
composed by He, we derived a distance to the source of 7.2$\pm$1.5 kpc.
