diff --git a/R/subsetting.R b/R/subsetting.R
index 4b1b4c985..e849482dd 100644
--- a/R/subsetting.R
+++ b/R/subsetting.R
@@ -636,35 +636,13 @@ is_tight_sequence_at_end <- function(i_new, n) {
 }
 
 tbl_subassign_col <- function(x, j, value) {
-  nrow <- fast_nrow(x)
-
-  # Grow, assign new names
-  new <- attr(j, "new")
-  if (!is.null(new)) {
-    length(x) <- max(j[new])
-    names(x)[j[new]] <- names2(j)[new]
+  if (length(j) > 1) {
+    order_j <- order(j)
+    value <- value[order_j]
+    j <- j[order_j]
   }
 
-  # Update
-  to_remove <- integer()
-  for (jj in seq_along(value)) {
-    ji <- j[[jj]]
-    value_jj <- value[[jj]]
-    if (!is.null(value_jj)) {
-      x[[ji]] <- value_jj
-    } else {
-      to_remove <- c(to_remove, ji)
-    }
-  }
-
-  # Remove
-  if (length(to_remove) > 0) {
-    x <- x[-to_remove]
-  }
-
-  # Can be destroyed by setting length
-  attr(x, "row.names") <- .set_row_names(nrow)
-  x
+  .Call(`tibble_tbl_subassign_col`, x, j, value)
 }
 
 tbl_expand_to_nrow <- function(x, i) {
diff --git a/src/init.c b/src/init.c
index 4588296a2..79c3ce574 100644
--- a/src/init.c
+++ b/src/init.c
@@ -9,6 +9,7 @@ static const R_CallMethodDef CallEntries[] = {
   {"tibble_string_to_indices", (DL_FUNC) &tibble_string_to_indices, 1},
   {"tibble_restore_impl", (DL_FUNC) &tibble_restore_impl, 2},
   {"tibble_need_coerce", (DL_FUNC) &tibble_need_coerce, 1},
+  {"tibble_tbl_subassign_col", (DL_FUNC) &tbl_subassign_col, 3},
 
   {NULL, NULL, 0}
 };
diff --git a/src/tbl_subassign_col.c b/src/tbl_subassign_col.c
new file mode 100644
index 000000000..184914a0b
--- /dev/null
+++ b/src/tbl_subassign_col.c
@@ -0,0 +1,131 @@
+#include "tibble.h"
+
+R_xlen_t fast_nrow(SEXP x);
+
+// Performs a variant of x[j] <- value, growing x as necessary
+// NULL values are supported
+// First, creates the storage for the resulting x
+// Then, populates that storage by copying the correct values exactly once
+//
+// Requires j to be sorted on input
+SEXP tbl_subassign_col(SEXP x, SEXP j_, SEXP value) {
+  // Naming conventions:
+  // n_x: length of x
+  // x_idx: index into vector x
+  // x_names: names of x
+  // n_old: number of elements with some property
+  // j_max: maximum value of j
+  // j_/j: SEXP and native version of bare vector
+  // xo: output vector
+  R_xlen_t nrow = fast_nrow(x);
+  R_xlen_t n_x = Rf_xlength(x);
+
+  // Compute j from j_, converting 1-based j_ to zero-based j
+  R_xlen_t n_j = Rf_xlength(j_);
+  R_xlen_t* j = (R_xlen_t*)R_alloc(n_j + 1, sizeof(R_xlen_t));
+  if (TYPEOF(j_) == INTSXP) {
+    const int* j_int = INTEGER(j_);
+    for (R_xlen_t j_idx = 0; j_idx < n_j; ++j_idx) {
+      j[j_idx] = j_int[j_idx] - 1;
+    }
+  } else if (TYPEOF(j_) == REALSXP) {
+    const double* j_real = REAL(j_);
+    for (R_xlen_t j_idx = 0; j_idx < n_j; ++j_idx) {
+      j[j_idx] = (R_xlen_t)j_real[j_idx] - 1;
+    }
+  } else {
+    Rf_error("Internal: tbl_subassign_col: invalid type for j_.");
+  }
+
+  // Add sentinel value
+  j[n_j] = -1;
+
+  // Compute j_max and n_old
+  R_xlen_t j_max = n_x;
+  R_xlen_t n_old = 0;
+  for (R_xlen_t j_idx = 0; j_idx < n_j; ++j_idx) {
+    if (j_max <= j[j_idx]) {
+      j_max = j[j_idx] + 1;
+    }
+    if (VECTOR_ELT(value, j_idx) == R_NilValue) {
+      ++n_old;
+    }
+  }
+
+  R_xlen_t n_xo = j_max - n_old;
+
+  // For each target element, position of source element in the x or value vectors
+  R_xlen_t* xo_src_idx = (R_xlen_t*)R_alloc(n_xo, sizeof(R_xlen_t));
+  bool* xo_is_value = (bool*)R_alloc(n_xo, sizeof(bool));
+
+  for (R_xlen_t j_idx = 0, xo_idx = 0, x_idx = 0; xo_idx < n_xo; ++x_idx) {
+    // Is the next xo element taken from x or from value?
+    bool is_value = (x_idx == j[j_idx]);
+    xo_is_value[xo_idx] = is_value;
+
+    if (is_value) {
+      // If the next value is not NULL, use it, otherwise skip it
+      if (VECTOR_ELT(value, j_idx) != R_NilValue) {
+        xo_src_idx[xo_idx++] = j_idx;
+      }
+      j_idx++;
+    } else {
+      // Use next x
+      xo_src_idx[xo_idx++] = x_idx;
+    }
+  }
+
+  // Allocate output vector and output names with final size
+  SEXP xo = Rf_allocVector(VECSXP, n_xo);
+  Rf_copyMostAttrib(x, xo);
+
+  SEXP xo_names = Rf_allocVector(STRSXP, n_xo);
+  SEXP j_names = Rf_getAttrib(j_, R_NamesSymbol);
+  SEXP x_names = Rf_getAttrib(x, R_NamesSymbol);
+
+  // Populate xo, copying from x or value as planned
+  for (R_xlen_t xo_idx = 0; xo_idx < n_xo; ++xo_idx) {
+    R_xlen_t x_idx = xo_src_idx[xo_idx];
+    if (xo_is_value[xo_idx]) {
+      SET_VECTOR_ELT(xo, xo_idx, VECTOR_ELT(value, x_idx));
+      SET_STRING_ELT(xo_names, xo_idx, STRING_ELT(j_names, x_idx));
+    } else {
+      if (x_idx >= n_x) {
+        Rf_error("Internal: tbl_subassign_col: x_j >= n_x, %d >= %d", x_idx, n_x);
+      }
+      SET_VECTOR_ELT(xo, xo_idx, VECTOR_ELT(x, x_idx));
+      SET_STRING_ELT(xo_names, xo_idx, STRING_ELT(x_names, x_idx));
+    }
+  }
+
+  // Set output names
+  Rf_setAttrib(xo, R_NamesSymbol, xo_names);
+
+  // Set output row names
+  // FIXME: Reuse original vector?
+  SEXP new_row_names_ = Rf_allocVector(INTSXP, 2);
+  int* new_row_names = INTEGER(new_row_names_);
+  new_row_names[0] = NA_INTEGER;
+  new_row_names[1] = -nrow;
+  Rf_setAttrib(xo, R_RowNamesSymbol, new_row_names_);
+
+  return xo;
+}
+
+R_xlen_t fast_nrow(SEXP x) {
+  SEXP row_names = Rf_getAttrib(x, R_RowNamesSymbol);
+
+  if (Rf_xlength(row_names) == 0) {
+    return 0;
+  }
+
+  if (TYPEOF(row_names) == INTSXP && INTEGER_ELT(row_names, 0) == NA_INTEGER) {
+    int out = INTEGER_ELT(row_names, 1);
+    if (out < 0) {
+      out = -out;
+    }
+    return out;
+  } else {
+    return Rf_xlength(row_names);
+  }
+}
diff --git a/src/tibble.h b/src/tibble.h
index b1c762fb2..5fb0f8061 100644
--- a/src/tibble.h
+++ b/src/tibble.h
@@ -4,10 +4,13 @@
 #define R_NO_REMAP
 #include <Rinternals.h>
 
+#include "stdbool.h"
+
 SEXP tibble_matrixToDataFrame(SEXP xSEXP);
 SEXP tibble_string_to_indices(SEXP x);
 SEXP tibble_need_coerce(SEXP x);
 SEXP tibble_update_attrs(SEXP x, SEXP dots);
 SEXP tibble_restore_impl(SEXP xo, SEXP x);
+SEXP tbl_subassign_col(SEXP x, SEXP j, SEXP value);
 
 #endif /* TIBBLE_H */