  We apply the Effective Field Theory of Large-Scale Structure (EFTofLSS) to
analyze cosmological models with clustering quintessence, which allows us to
consistently describe the parameter region in which the quintessence equation
of state $w < - 1$. First, we extend the description of biased tracers in
redshift space to the presence of clustering quintessence, and compute the
one-loop power spectrum. We solve the EFTofLSS equations using the exact time
dependence, which is relevant to obtain unbiased constraints. Then, fitting the
full shape of BOSS pre-reconstructed power spectrum measurements, the BOSS
post-reconstruction BAO measurements, BAO measurements from 6DF/MGS and eBOSS,
the Supernovae from Pantheon, and a prior from BBN, we bound the clustering
quintessence equation of state parameter $w=-1.011_{-0.048}^{+0.053}$ at $68\%$
C.L.. Further combining with Planck, we obtain $w=-1.028_{-0.030}^{+0.037}$ at
$68\%$ C.L.. We also obtain constraints on smooth quintessence, in the physical
regime $w \geq -1$: combining all datasets, we get $-1\leq w < - 0.979$ at
$68\%$ C.L.. These results strongly support a cosmological constant.
